Bump actions/cache from 1 to 2.1.6

Bumps [actions/cache](https://github.com/actions/cache) from 1 to 2.1.6.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v1...v2.1.6)

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build.yml

@@ -25,19 +25,19 @@ jobs:
           override: true
 
       - name: Cache cargo registry
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: ~/.cargo/registry
           key: ${{ runner.os }}-v1-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Cache cargo index
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: ~/.cargo/git
           key: ${{ runner.os }}-v1-cargo-index-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Cache cargo target dir
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: target
           key: ${{ runner.os }}-v1-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
@@ -64,19 +64,19 @@ jobs:
           override: true
 
       - name: Cache cargo registry
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: ~/.cargo/registry
           key: ${{ runner.os }}-v1-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Cache cargo index
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: ~/.cargo/git
           key: ${{ runner.os }}-v1-cargo-index-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Cache cargo target dir
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: target
           key: ${{ runner.os }}-v1-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}
@@ -104,19 +104,19 @@ jobs:
           components: rustfmt, clippy
 
       - name: Cache cargo registry
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: ~/.cargo/registry
           key: ${{ runner.os }}-v1-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Cache cargo index
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: ~/.cargo/git
           key: ${{ runner.os }}-v1-cargo-index-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Cache cargo target dir
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: target
           key: ${{ runner.os }}-v1-cargo-build-target-${{ hashFiles('**/Cargo.lock') }}