Make the BUG_REPORT_URL configurable by tools
This greatly simplifies how hard it is to set a custom bug report url; previously tools had to copy
the entire hook implementation.
I haven't changed clippy in case they want to make the change upstream instead of the subtree, but
I'm happy to do so here if the maintainers want - cc ````@rust-lang/clippy````
Fixes https://github.com/rust-lang/rust/issues/109486.