doc: add build instructions for musl-libc (#5572)

book/src/install.md

@@ -99,6 +99,14 @@ cargo install --path helix-term --locked
 
 This will install the `hx` binary to `$HOME/.cargo/bin` and build tree-sitter grammars in `./runtime/grammars`.
 
+If you are using the musl-libc instead of glibc the following environment variable must be set during the build
+to ensure tree sitter grammars can be loaded correctly:
+
+```
+RUSTFLAGS="-C target-feature=-crt-static"
+```
+
+
 Helix also needs its runtime files so make sure to copy/symlink the `runtime/` directory into the
 config directory (for example `~/.config/helix/runtime` on Linux/macOS). This location can be overridden
 via the `HELIX_RUNTIME` environment variable.