Pin used proc-macro-srv nightly version heuristically?

[Veykril]

          Looks like a rustc sync is needed after https://github.com/rust-lang/rust/pull/139671.

Originally posted by @Kobzol in #19597 (comment)

The proc-macro-srv CI step can be quite annoying when changes happen upstream regarding libproc-macro. I wonder if we can smartly pin the nightly used to the last synced rust-lang/rust version?

[Veykril]

@lnicola maybe you got some ideas here given you know how syncs go from usually. Maybe it suffices to just bump the nightly used in CI to the nightly of when you do a sync? (iirc we use an xtask for syncing so that bump can be done automatigcally with that)

[Kobzol]

One issue with that is that the synced commit might not necessarily correspond to a nightly (we could find the nearest nightly, I suppose, but that might be "in the future" at the time of sync). Another alternative is to use https://github.com/kennytm/rustup-toolchain-install-master and just install the commit from rust-version.

[Kobzol]

Trying that in #19600.

[Kobzol]

Looks like it works :)

[Veykril]

Oh sweet, we can in fact use the rust-version file. I was wondering about that.