Add lint check for executable source files

As demonstrated by !2300 (merged), none of the CI linting jobs currently check for source files (e.g. *.c, *.py) that have executable bits set. It would be good to add such a check.

@adam-mercer @duncanmmacleod Off the top of your lead, do any of our current linting tools have the ability to do such a check? Or would we need to add another tool?