Fix Python caching in the GitHub Actions workflows

For the Python-based workflows we were already using `pip` caching [1],
but sadly this isn't fully functional at the moment because the caching
functionality uses `requirements.txt` to determine when to create or
invalidate the cache. However, we have two different `pip` install
commands but only a `requirements.txt` for one of them (the Fluent
linter), which means that the other job (the font tests) will not
populate the cache with its dependencies.

This can be seen by opening any font tests or Fluent linting build and
noticing that they report the exact same cache key even though their
dependencies are different. In the installation step the dependencies
are reported as "Downloading [package].whl" instead of the expected
"Using cached [package].whl".

This commit fixes the issue by explicitly defining a `requirements.txt`
file for both jobs and pointing the caching functionality to the
specific file paths to make sure that unique caches with the correct
package data are used. While we're here we also align the syntax and
step titles in the files for consistency.

[1] https://github.com/actions/setup-python?tab=readme-ov-file#caching-packages-dependencies

.github/font_tests_requirements.txt

@@ -0,0 +1 @@
+fonttools==4.*

.github/workflows/fluent_linter.yml

@@ -34,11 +34,10 @@ jobs:
         with:
           python-version: '3.14'
           cache: 'pip'
+          cache-dependency-path: '.github/fluent_linter_requirements.txt'
 
-      - name: Install Fluent dependencies
-        run: |
-          pip install -r .github/requirements.txt
+      - name: Install requirements
+        run: pip install -r .github/fluent_linter_requirements.txt
 
       - name: Lint Fluent reference files
-        run: |
-          moz-fluent-lint ./l10n/en-US --config .github/fluent_linter_config.yml
+        run: moz-fluent-lint ./l10n/en-US --config .github/fluent_linter_config.yml

.github/workflows/font_tests.yml

@@ -56,9 +56,10 @@ jobs:
         with:
           python-version: '3.14'
           cache: 'pip'
+          cache-dependency-path: '.github/font_tests_requirements.txt'
 
-      - name: Install Fonttools
-        run: pip install fonttools
+      - name: Install requirements
+        run: pip install -r .github/font_tests_requirements.txt
 
       - name: Run font tests
         run: npx gulp fonttest --headless