ci: compare Bicep file in PR to Bicep file in base branch (#20)

Resolves issue where Bicep file is not built even though changes have been made.

.github/workflows/build.yml

@@ -16,13 +16,14 @@ jobs:
       - name: Checkout
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
         with:
-          ref: ${{ github.head_ref }} # Required to push commit to PR
+          fetch-depth: 0 # Required to show diff between PR source and target branch
+          ref: ${{ github.head_ref }} # Required to push commit to PR source branch
 
       - name: Build Bicep file
         id: build
         shell: bash {0} # Required to check exit code
         run: |
-          git diff --exit-code "$BICEP_FILE_PATH"
+          git diff --exit-code "origin/$GITHUB_BASE_REF" "$BICEP_FILE_PATH"
           exit_code="$?"
           if [[ "$exit_code" == 0 ]]; then
             echo "No changes made to Bicep file. Skipping build."