Detect and retry "checksum failed"

The `rustup toolchain install` step occasionally fails like this.

```console
info: syncing channel updates for beta-x86_64-pc-windows-msvc
warn: checksum failed for 'https://static.rust-lang.org/dist/channel-rust-beta.toml', expected: '7c8035eccd259661acc845afb33fb40892f4722327f7d8d8c49feda172d1a159', calculated: 'e8c5f7bc05a9516072acb06d4da8ec4dd0722591a7fe4c5df4628be3c0de0fb4'
info: this is likely due to an ongoing update of the official release server, please try again later
info: see for more details
error: toolchain 'beta-x86_64-pc-windows-msvc' is not installable
```

It might be reasonable for this action to automatically sleep and retry on this specific error a few times (as much as a couple minutes).

Start at the action entry point that runs `rustup toolchain install` and inspect how its output and exit status are handled. Reproduce the shown checksum-failed output, then verify that only this error triggers bounded retries and that a successful retry completes installation.

Written by the indexing model from the issue text.