github: drop cache

this doesn't seem to speed things up much, so let's remove it for now

.github/workflows/test-pull-request.yml

@@ -14,16 +14,6 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Get yarn cache directory path
-      id: yarn-cache-dir-path
-      run: echo "::set-output name=dir::$(yarn cache dir)"
-    - uses: actions/cache@v1
-      id: yarn-cache
-      with:
-        path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
-        key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
-        restore-keys: |
-          ${{ runner.os }}-yarn-
     - name: Run yarn install
       run: |
         echo "//npm.pkg.github.com/:_authToken=$TOKEN" >> $HOME/.npmrc