update rust-gpu to PR #545

[Firestar99]

• update rust-gpu to Update toolchain nightly-2026-03-14 rust-gpu#545
  • spirv-builder changes in that PR may cause old toolchains to fail? yep, needed adjusting
• add cargo xtask rust-gpu-rev <rev> to automate rust-gpu dep upgrading
• use expect-test crate (from rust-analyzer project) to update expected strings in tests