Fix Rust Cache invalid key warning (#59)

The `Swatinem/rust-cache` GitHub action does not support commas
in its cache key, but we use the Cargo feature set (which can have commas)
as part of the cache key. This commit hashes the feature set to avoid this
problem.

.github/workflows/ci.yml

@@ -43,10 +43,17 @@ jobs:
           # https://github.com/actions/runner/issues/409#issuecomment-752775072
           components: ${{ contains(matrix.channel, 'nightly') && 'miri' || '' }}
 
+    # The features string contains commas which cannot be part of the cache
+    # key for the Rust Cache action. Instead, we hash the features
+    # to get a string of legal characters.
+    - name: Set feature string for cache key
+      run: |
+        echo "FEATURES_HASH=$(echo ${{ matrix.features }} | sha256sum | cut -d ' ' -f 1)" >> $GITHUB_ENV
+
     - name: Rust Cache
       uses: Swatinem/rust-cache@v2.0.0
       with:
-        key: "${{ matrix.channel }}-${{ matrix.target }}-${{ matrix.features }}-${{ hashFiles('**/Cargo.lock') }}"
+        key: "${{ matrix.channel }}-${{ matrix.target }}-${{ env.FEATURES_HASH }}-${{ hashFiles('**/Cargo.lock') }}"
 
     - name: Check
       run: cargo +${{ matrix.channel }} check --target ${{ matrix.target }} --features "${{ matrix.features }}" --verbose