Set Rust caching key per language group (#2250)

Fixes #2247 (a follow-up from #2195).

* Add a workflow dispatch manual trigger to the tests workflow.
* Move the link-checked languages JSON list to the build job's env.
* Set a caching key per language group to avoid extra `mdbook-linkcheck`
installation on non-link-checked translations.
  For the meaning of this key, see:
https://github.com/Swatinem/rust-cache?tab=readme-ov-file#example-usage
* Fix newly broken web link in 8a3ed21 ✅ 

I tested it on my fork, and it looks good:
https://github.com/jond01/comprehensive-rust/actions/workflows/build.yml
https://github.com/jond01/comprehensive-rust/actions/caches

.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:
@@ -9,6 +15,7 @@ runs:
       uses: Swatinem/rust-cache@v2
       with:
         # Only save the cache on the main branch to avoid PRs filling
-        # up the cache. Further, only save it if we are working on the
-        # English source (or if no language has been set).
-        save-if: ${{ github.ref == 'refs/heads/main' && (matrix.language == 'en' || matrix.language == '') }}
+        # up the cache.
+        save-if: ${{ github.ref == 'refs/heads/main' }}
+        # Further, save the cache per key - e.g. language grouping.
+        key: ${{ inputs.key }}