tools: work around breaking change in rustup 1.28.0

[iliana]

This works around rust-lang/rustup#3985. Tested via Omicron's cargo xtask releng.

[sunshowers]

Do we want to capture and log this instead?

[iliana]

In an ideal world where I'm not trying to perform an emergency fix to Omicron CI, maybe!

[iliana]

The output is not particularly useful, it just shows the current toolchain and where the override comes from, if any. The stderr is left in place, so that's logged to the console whenever an error occurs (including when the toolchain is not installed).

[sunshowers]

Totally makes sense

[jclulow]

These things have a habit of sticking around so I did the logging part and pushed it onto the branch. Does that look alright?

It produces log messages that look like:

Mar 03 19:04:42.036 INFO building project "image-builder" ok (0 seconds)
Mar 03 19:04:42.036 INFO checking rust toolchain is installed for "/ws/helios/projects/pinprick"
Mar 03 19:04:42.063 INFO rust toolchain for "/ws/helios/projects/pinprick": "stable-x86_64-unknown-illumos\nactive because: it's the default toolchain"
Mar 03 19:04:42.063 INFO building project "pinprick" at /ws/helios/projects/pinprick
Mar 03 19:04:42.063 INFO exec: ["cargo", "build", "--locked", "--release"], pwd: Some("/ws/helios/projects/pinprick")

[iliana]

Looks fine to me! Apparently I'm not allowed to approve my own PR and new changes require approval from someone other than the last pusher.

[jclulow]

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