Build/Test Tools: Correctly test grunt clean when built to run from src.

Because the `--` separator was missing, the `--dev` flag was not being passed to the script, thus not testing properly.

See #52625.

git-svn-id: https://develop.svn.wordpress.org/trunk@50796 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/test-npm.yml

@@ -114,7 +114,7 @@ jobs:
         run: npm run build:dev
 
       - name: Clean after building in /src
-        run: npm run grunt clean --dev
+        run: npm run grunt clean -- --dev
 
   # Verifies that installing NPM dependencies and building WordPress works as expected on MacOS.
   #
@@ -172,4 +172,4 @@ jobs:
         run: npm run build:dev
 
       - name: Clean after building in /src
-        run: npm run grunt clean --dev
+        run: npm run grunt clean -- --dev