Handle RUNFILES_DIR and RUNFILES_MANIFEST_FILE both

[a-zw]

Bazel on Linux usually sets the RUNFILES_DIR environment variable but it may alternatively set RUNFILES_MANIFEST_FILE. General advice for scripts is to make your scripts capable of working with both.

At least src/extensions/score_plantuml.py does not handle RUNFILES_MANIFEST_FILE.

[a-zw]

I have no idea how trigger the behaviors reliably, so asked on SO.