Add stability to Composer Cache

I'm not 100 sure on this one but I think the Cache dependencies key should include the stability (prefer-lowest or prefer-stable).

Otherwise, some of the dependencies in the prefer-lowest build seem to be re-downloaded every time -  unless the lowest version matches the stable one.

If I'm correct this should increase the speed of every build.

Author: sileence

.github/workflows/tests.yml

@@ -40,7 +40,7 @@ jobs:
         uses: actions/cache@v1
         with:
           path: ~/.composer/cache/files
-          key: dependencies-php-${{ matrix.php }}-composer-${{ hashFiles('composer.json') }}
+          key: dependencies-php-${{ matrix.php }}-${{ matrix.stability }}-composer-${{ hashFiles('composer.json') }}
 
       - name: Setup PHP
         uses: shivammathur/setup-php@v2