Use a true/false key in the build cache

Reduce the number of caches to only two.

.github/workflows/build.yml

@@ -132,6 +132,8 @@ jobs:
 
       - name: Setup Rust cache
         uses: ./.github/workflows/setup-rust-cache
+        with:
+          key: ${{ contains(fromJSON(env.LINK_CHECKED_LANGUAGES), matrix.language) }}
 
       - name: Install Gettext
         run: |

.github/workflows/setup-rust-cache/action.yml

@@ -2,6 +2,12 @@ name: Setup Rust cache
 
 description: Configure the rust-cache workflow.
 
+inputs:
+  key:
+    description: Additional caching key
+    required: false
+    default:
+
 runs:
   using: composite
   steps:
@@ -11,5 +17,5 @@ runs:
         # Only save the cache on the main branch to avoid PRs filling
         # up the cache.
         save-if: ${{ github.ref == 'refs/heads/main' }}
-        # Further, save the cache per language.
-        key: ${{ matrix.language }}
+        # Further, save the cache per key - e.g. language grouping.
+        key: ${{ inputs.key }}