ci: Use -C rather than -Z for instrument-coverage. (#332)

Using `-C` was stabilized in Rust 1.60.

Author: waywardmonkeys

.github/workflows/test.yaml

@@ -49,7 +49,7 @@ jobs:
       - name: Run tests
         run: cargo test --all-features --no-fail-fast
         env:
-          RUSTFLAGS: '-Zinstrument-coverage'
+          RUSTFLAGS: '-Cinstrument-coverage'
       - name: Run grcov
         if: matrix.rust-version == 'nightly' && matrix.cargo-args == '--all-features'
         # Important! Keep in grcov flags in sync with Makefile.internal.toml.