Per code review for #57257, we should make the following improvements to our bash entrypoints:

1. Ensure follow_links() (in //bin/flutter and //bin/dart) never emits double slashes.
2. Replace shlock with http://mywiki.wooledge.org/BashFAQ/045 in //bin/shared.sh.
3. shared.sh does not need executable permissions.

cc @gspencergoog