chore(ci): adjust validation of test changes

borrowed from python-docs-samples https://github.com/GoogleCloudPlatform/python-docs-samples/blob/a163a4247af1ccdcb6138756e0adf75604069326/.kokoro/tests/run_tests.sh#L35

Author: glasnt

build/int.cloudbuild.yaml

@@ -35,12 +35,12 @@ steps:
       cat _changed_folders
 
       # Do not prune if changing tests themselves
-      while read d; do
-        if [[ "build test .github/workflows" =~ "${d%/}" ]]; then
-          echo "Infrastructure folder ${d%/} has changed; no tests will be pruned."
+      git diff --quiet origin/main build test .github/workflows
+      CHANGED=$?
+      if [[ "${CHANGED}" -ne 0 ]]; then
+          echo "Infrastructure folders have changed; no tests will be pruned."
           exit 0 # do not prune
-        fi
-      done < _changed_folders
+      fi
 
       # Remove base folders without changes
       for d in *; do