Add prefer-lowest to composer cache key

.github/workflows/ci.yml

@@ -176,7 +176,7 @@ jobs:
       uses: actions/cache@v1
       with:
         path: ${{ steps.composer-cache.outputs.dir }}
-        key: ${{ runner.os }}-composer-${{ steps.key-date.outputs.date }}-${{ hashFiles('composer.json') }}
+        key: ${{ runner.os }}-composer-${{ steps.key-date.outputs.date }}-${{ hashFiles('composer.json') }}-${{ matrix.prefer-lowest }}
 
     - name: composer install
       run: composer install
@@ -215,7 +215,7 @@ jobs:
       uses: actions/cache@v1
       with:
         path: ${{ steps.composer-cache.outputs.dir }}
-        key: ${{ runner.os }}-composer-${{ steps.key-date.outputs.date }}-${{ hashFiles('composer.json') }}
+        key: ${{ runner.os }}-composer-${{ steps.key-date.outputs.date }}-${{ hashFiles('composer.json') }}-${{ matrix.prefer-lowest }}
 
     - name: composer install
       run: composer stan-setup