Dynamically figure out the actual path to Composer's binary, and make it read-only

This seems like a reasonable approach to me. I was thinking we'd want to do it on composer install/update ... somewhere... , but since we're already checking file_exists(), checking is_executable() at the same time isn't much more work.

As a follow-up to ✨ Package Manager should use a copy of Composer that is local to the current project, if available Active this makes sense

Couple of comments on the MR.

Switched those to non-static class properties. I'm going to boldly restore the RTBC here because it wasn't a substantive logic change, and reviewer time is quite short these days, and I'm really hoping to get this backported into 11.2.3. :)

Committed/pushed to 11.x and cherry-picked to 11.2.x, thanks!

catch → closed merge request !12822