x/build/cmd/coordinator: add ability to auto-add slowbots based on filepaths modified in a CL

[mdempsky]

On the dev.regabi branch, there's naturally a lot of development happening related to the regabi experiment. It would be nice if the linux-amd64-regabi slowbot was automatically included in Trybot runs for that branch.

Similarly, a lot of this work is happening in the SSA backend, and concurrency issues here are only caught on the linux-amd64-racecompile slowbot. It would be nice if this slowbot could either also be added by default for dev.regabi, or maybe for all CLs that touch src/cmd/compile.

I've broken these builders 3 times in the past ~week. 😬

/cc @dmitshur

[cagedmantis]

/cc @golang/release

[bcmills]

Compare #42661.

[dmitshur]

I'll retitle this to about adding the ability to opt-in to builders based on files modified in a CL. We need to do some work for it, but it would enable us to improve pre-submit test coverage in a more targeted way than currently possible. It will help make progress on issues like #42661 and #19535.

We already have a mechanism to enable trybots based on branch name by specifying a custom tryBot policy function—no new work is needed for this. @mdempsky If there are TryBot changes that are worth making with branch-name filtering only, please let me know (I suspect the branch situation has changed since this issue was originally filed).