[ci] Roll pinned nightly toolchain

[google-pr-creation-bot]

Automated changes by create-pull-request GitHub action

[jswrenn]

The CI breakage was caused by syn bumping its MSRV to 1.60. The timeline of events is a little confusing:

1. March 21: Raise required compiler to rust 1.60
2. March 23: Release 2.0.54
3. March 23: Restore compatibility with rustc 1.56 through 1.59
   
4. March 23: Release 2.0.55
5. March 29: chore: bump msrv to 1.60 in Use dep: and optional dependency feature ? syntax dtolnay/syn#1608
6. March 29: Release 2.0.56

Not yet sure how we should proceed with this. A few possibilities that spring to mind:

• We build in CI with -Z minimal-versions
• We downgrade to syn 1.x (MSRV is 1.31)
• We vendor (or reimplement) the parts of syn relevant to zerocopy-derive
• We delete our MSRV policy on zerocopy-derive; we cannot guarantee an MSRV for that crate, since it has dependencies with different MSRV policies.

[joshlf]

Obsoleted by #1090