Revert "[ci] Diff against main when determining what files have changed for pre-commit CI (#67743)"

This reverts commit ec9d80e, since we are currently seeing tons of
CI jobs being triggered spuriously.

Author: ldionne

.ci/generate-buildkite-pipeline-premerge

@@ -22,7 +22,7 @@ set -o pipefail
 
 # Environment variables script works with:
 # List of files affected by this commit
-: ${MODIFIED_FILES:=$(git diff --name-only main...HEAD)}
+: ${MODIFIED_FILES:=$(git diff --name-only HEAD~1)}
 # Filter rules for generic windows tests
 : ${WINDOWS_AGENTS:='{"queue": "windows"}'}
 # Filter rules for generic linux tests