TUF repo build failed on phbl Rust toolchain error

This build failed in CI on #7975 but I don't know why this would have to do with my changes:
https://github.com/oxidecomputer/omicron/pull/7975/checks?check_run_id=40616188430

Here's the failure in the log:
https://buildomat.eng.oxide.computer/wg/0/details/01JRXRMBDSZCCCXNBCR15SYDV5/r5B6t7VRgsMcqy6RsbDQGvj8hVom6kidHwsWhdHMvbLkfNb2/01JRXRMZ15CD2CESFEYWRM0KB3#S9966

Excerpt from the log showing the failure:

9964	2025-04-15T23:15:08.044Z	[  recovery-image] Apr 15 23:15:06.588 INFO rust toolchain for "/work/oxidecomputer/helios/projects/phbl": "nightly-2025-04-02-x86_64-unknown-illumos (overridden by '/work/oxidecomputer/helios/projects/phbl/rust-toolchain.toml')"
9965	2025-04-15T23:15:08.045Z	[  recovery-image] Apr 15 23:15:06.588 INFO exec: ["cargo", "xtask", "build", "--release", "--cpioz", "/work/os-recovery/cpio.z"], pwd: Some("/work/oxidecomputer/helios/projects/phbl")
9966	2025-04-15T23:15:11.442Z	[  recovery-image] Apr 15 23:15:09.986 INFO E| error: the 'cargo' binary, normally provided by the 'cargo' component, is not applicable to the 'nightly-2025-04-02-x86_64-unknown-illumos' toolchain

Start with the phbl/rust-toolchain.toml entry named in the build log and the cargo xtask build invocation from the linked CI run. Determine why cargo is unavailable for nightly-2025-04-02-x86_64-unknown-illumos, then rerun the TUF repository build to confirm the recovery-image step succeeds.