ci: run tests in parallel

This will run the three test suites (unit, functional and old) in
parallel, meaning that neovim is built for each test and run separately.
This has a slight increase in total CI usage, but it allows rerunning
only the specific test suite that failed for flaky tests, which will
save some time.

Ideally we'd remove any drawbacks by building neovim once and reusing it
for each test suite, but that is not currently possible due to poor
upload/download speeds of the upload-artifact and download-artifact
actions. This has been addressed in
https://github.com/actions/toolkit/pull/1488, but will only be made
available in upload-artifact@v4 and download-artifact@v4.

.github/actions/cache/action.yml

@@ -10,6 +10,10 @@ runs:
       run: echo "CACHE_KEY=$CACHE_KEY-${{ join(matrix.*, '-') }}" >> $GITHUB_ENV
       shell: bash
 
+    - if: ${{ matrix.build }}
+      run: echo "CACHE_KEY=$CACHE_KEY-${{ join(matrix.build.*, '-') }}" >> $GITHUB_ENV
+      shell: bash
+
     - id: image
       run: echo "version=$ImageVersion" >> $GITHUB_OUTPUT
       shell: bash