By default, we make every rustup user use our pinned version. This might
not be ideal at this point, for a few reasons:
1. we don't have automatic Rust updates yet (see the parent commit),
so this might unnecessarily install an old version. As a contributor,
this feels irritating (newer versions are usually strictly better).
2. it will use more bandwidth and perhaps other resources during "git-bisect"
scenarios
3. somehow rustup will download things redundantly; it will download "1.89.0"
and "stable" even if they are identical. The user will need to clean
those up at some point, even if they didn't add them explicitly.
See also
https://github.com/fish-shell/fish-shell/pull/11712#issuecomment-3165388330
Part of the motivation for rust-toolchain.toml is probably the regular
(every 6 weeks) failures due to the update check, but that failure has been
removed in the parent commit.
The other motivation ("fix the issue of local compiles running into lint
warnings from newer compilers") is a fair point but I think we should rather
fix warnings quickly.
Let's remove rust-toolchain.toml again until we have more agreement on what
we should do.
This reverts commits
* f806d35af8 (Ignore rust-toolchain.toml in CI, 2025-08-07)
* 9714b98262 (Explicitly use fully qualified rust version numbers, 2025-08-07)
* 921aaa0786 (Add rust-toolchain.toml, 2025-08-07)
Closes#11718
Extract a github action to reduce the number of references to our MSRV and
stable (to be pinned in the next commit).
While at it, use the MSRV for macOS builds; this means that we'll be less
like accidentally to break the macOS build when bumping the MSRV. I don't
think there is a reason for using 1.73 specifically, other than "it's the
highest we can use on old macOS", so using an even older one should be fine.
