Revert to head and fetch

Author: llrs-roche

.github/workflows/build-check-install.yaml

@@ -543,7 +543,7 @@ jobs:
 
           # Get the list of R files that have changed
           # https://github.com/cloudnativeto/envoy/blob/de67446fa8af973761bb4716f5143d73643baf8b/.github/workflows/get_build_targets.sh_bak#L9
-          files=$(git diff --name-only main.. -- $test_dir R/*.R)
+          files=$(git diff --name-only HEAD FETCH_HEAD -- $test_dir R/*.R)
 
           if [ -z "$files" ]
           then {