ci: Fix for rustup breakage

[notpeter]

Something broke with GitHub actions runners in the last two days.
This is an attempt to fix it.

Current main 918848c passed May 12, 2026, 12:05 PM EDT
https://github.com/oxidecomputer/dropshot/actions/runs/25746385530/job/75610647928

But failed when I reran it today May 14, 2026, 10:19 AM EDT
https://github.com/oxidecomputer/dropshot/actions/runs/25746385530/job/76005483181

Error is:

Run cargo build --all-features --locked --all-targets --verbose
error: error: unexpected argument 'build' found

Usage: rustup-init[EXE] [OPTIONS]

For more information, try '--help'.

Which makes me think there's a rustup-init cargo alias or somesuch.
I didn't see any upstream issues for this on rust-lang/rustup or actions/runner

Found it:

• cache-bin: true breaks cargo subcommands on new github macos runner Swatinem/rust-cache#341

Root cause is the changes in macos-14-arm64 (20260512) Image Update.

[ahl]

Is this a workaround that we expect to be temporary?

[notpeter]

Is this a workaround that we expect to be temporary?

I would expect yes. The logic changed here: Swatinem/rust-cache@ed5e89b (2026-04-12) so it would cache everything in $CARGO_HOME/bin. Swatinem/rust-cache#341 links to changes for the macos action runner between 20260427.0019.1 and 20260512.0058.1 but I don't see anything mention that should've broken this.

My expectation is that either Swatinem/rust-cache or the official GitHubs images (or both) will make things not broken by default. (the current state)