ci: configure restore-keys for caches (#1806)

`restore-keys` is a configuration option for the actions/cache action
which specifies fallback behavior. The [docs][docs] say it best:

> When a cache miss occurs, the action searches for alternate keys
> called `restore-keys`.
>
> If you provide `restore-keys`, the `cache` action sequentially
> searches for any caches that match the list of `restore-keys`.
> ... If there are no exact matches, the action searches for partial
> matches of the restore keys. When the action finds a partial match,
> the most recent cache is restored to the `path` directory.

So this improves caching when there's a miss. For example if I edit
`.github/workflows/languages.toml`, the current behavior is that the
cache for downloaded grammars will miss and all of them will need to
be fetched again. With `restore-keys`, we use the latest published
cache as 'good enough', we'll fetch whatever grammars changed, and
then at the end we publish a new cache under the new hash.

[docs]: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#example-using-the-cache-action

.github/workflows/build.yml

@@ -27,18 +27,21 @@ jobs:
         with:
           path: ~/.cargo/registry
           key: ${{ runner.os }}-v2-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-registry-
 
       - name: Cache cargo index
         uses: actions/cache@v2.1.7
         with:
           path: ~/.cargo/git
           key: ${{ runner.os }}-v2-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-index-
 
       - name: Cache cargo target dir
         uses: actions/cache@v2.1.7
         with:
           path: target
           key: ${{ runner.os }}-v2-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-build-target-
 
       - name: Run cargo check
         uses: actions-rs/cargo@v1
@@ -64,18 +67,21 @@ jobs:
         with:
           path: ~/.cargo/registry
           key: ${{ runner.os }}-v2-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-registry-
 
       - name: Cache cargo index
         uses: actions/cache@v2.1.7
         with:
           path: ~/.cargo/git
           key: ${{ runner.os }}-v2-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-index-
 
       - name: Cache cargo target dir
         uses: actions/cache@v2.1.7
         with:
           path: target
           key: ${{ runner.os }}-v2-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-build-target-
 
       - name: Copy minimal languages config
         run: cp .github/workflows/languages.toml ./languages.toml
@@ -85,6 +91,7 @@ jobs:
         with:
           path: runtime/grammars
           key: ${{ runner.os }}-v2-tree-sitter-grammars-${{ hashFiles('languages.toml') }}
+          restore-keys: ${{ runner.os }}-v2-tree-sitter-grammars-
 
       - name: Run cargo test
         uses: actions-rs/cargo@v1
@@ -117,18 +124,21 @@ jobs:
         with:
           path: ~/.cargo/registry
           key: ${{ runner.os }}-v2-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-registry-
 
       - name: Cache cargo index
         uses: actions/cache@v2.1.7
         with:
           path: ~/.cargo/git
           key: ${{ runner.os }}-v2-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-index-
 
       - name: Cache cargo target dir
         uses: actions/cache@v2.1.7
         with:
           path: target
           key: ${{ runner.os }}-v2-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-build-target-
 
       - name: Run cargo fmt
         uses: actions-rs/cargo@v1
@@ -161,18 +171,21 @@ jobs:
         with:
           path: ~/.cargo/registry
           key: ${{ runner.os }}-v2-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-registry-
 
       - name: Cache cargo index
         uses: actions/cache@v2.1.6
         with:
           path: ~/.cargo/git
           key: ${{ runner.os }}-v2-cargo-index-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-index-
 
       - name: Cache cargo target dir
         uses: actions/cache@v2.1.6
         with:
           path: target
           key: ${{ runner.os }}-v2-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-v2-cargo-build-target-
 
       - name: Generate docs
         uses: actions-rs/cargo@v1