DEV: Use `--frozen-lockfile` in GitHub CI (#21338)

This will help us to catch missing lockfile changes before they are merged into `main`

.github/workflows/linting.yml

@@ -65,7 +65,7 @@ jobs:
             ${{ runner.os }}-yarn-
 
       - name: Yarn install
-        run: yarn install
+        run: yarn install --frozen-lockfile
 
       - name: Rubocop
         if: ${{ !cancelled() }}

.github/workflows/tests.yml

@@ -98,7 +98,7 @@ jobs:
             ${{ runner.os }}-yarn-
 
       - name: Yarn install
-        run: yarn install
+        run: yarn install --frozen-lockfile
 
       - name: Checkout official plugins
         if: matrix.target == 'plugins'
@@ -272,7 +272,7 @@ jobs:
 
       - name: Yarn install
         working-directory: ./app/assets/javascripts/discourse
-        run: yarn install
+        run: yarn install --frozen-lockfile
 
       - name: Ember Build
         working-directory: ./app/assets/javascripts/discourse