ci: Temporarily pin to older nightly

[nicholasbishop]

There are a bunch of new lint errors that need to be addressed before we can successfully build with latest nightly.

(Note that the +nightly is dropped here from the build_standard_uefi_binary job, because it overrides the toolchain specified in rust-toolchain.toml.)

#1687

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[nicholasbishop]

Merging this to unblock CI