build: fix changed-file-validation to allow PRs from forks

.github/workflows/default.yml

@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@v2
       - name: Validate changed files
         if: github.event_name == 'pull_request' && !contains(github.event.pull_request.labels.*.name, 'allow sdk change')
-        run: ./.github/workflows/validate_changed_files.sh ${{ github.head_ref }} ${{ github.base_ref }}
+        run: ./.github/workflows/validate_changed_files.sh
       - uses: bazelbuild/setup-bazelisk@v1
       - name: Bazel caches
         uses: actions/cache@v2

.github/workflows/validate_changed_files.sh

@@ -7,10 +7,11 @@ VALID_FILE_PATHS=('pkg/plugin/sdk/v1alpha2') # Array of filepaths that are allow
 
 # by default the only local branch will be pull/PR#/merge
 # fetch only the latest commit from the 2 branches in question to avoid fetching the entire repo which could be costly
-git fetch --depth 1 origin "$1"
-git fetch --depth 1 origin "$2"
 
-git diff "origin/$1..origin/$2" --name-only | while read -r file; do
+git fetch --depth 1 origin "${GITHUB_BASE_REF}"
+git fetch --depth 1 origin "${GITHUB_REF}"
+
+git diff --name-only "origin/${GITHUB_BASE_REF}..${GITHUB_SHA}" | while read -r file; do
 
     # check if filepath matches a valid path. If so move to the next change
     for valid_path in "${VALID_FILE_PATHS[@]}"; do