CI flake: rust-cache timeout

[jgallagher]

https://github.com/oxidecomputer/omicron/actions/runs/5871331819/job/15920417547 failed on the rust-cache step:

...
Received 761198491 of 765392795 (99.5%), 7.4 MBs/sec
Received 761198491 of 765392795 (99.5%), 7.3 MBs/sec
Received 761198491 of 765392795 (99.5%), 7.3 MBs/sec
Received 761198491 of 765392795 (99.5%), 7.2 MBs/sec
Received 761198491 of 765392795 (99.5%), 7.2 MBs/sec
Warning: Failed to restore: The operation cannot be completed in timeout.
No cache found.
Error: The operation was canceled.

This appears to be a fault upstream of the rust-cache action itself; Swatinem/rust-cache#72 has discussions about adding timeouts/retries, but it doesn't look like anything has landed yet.