Flake fetching crate metadata in minimal versions tests

[github-actions]

The Rust CI failed on the main branch.

Failure in Google Cloud Build:

• Check Suite: https://api.github.com/repos/googleapis/google-cloud-rust/check-suites/73792682602
• Commit: 3185f7a

[github-actions]

@googleapis/cloud-sdk-rust-team A critical issue has been created, please respond immediately.

[dbolduc]

error: failed to load source for dependency `windows-implement`
Caused by:
  unable to update registry `crates-io`
Caused by:
  download of wi/nd/windows-implement failed
Caused by:
  curl failed
Caused by:
  [16] Error in the HTTP2 framing layer
error: process didn't exit successfully: `/usr/local/rustup/toolchains/1.96.0-x86_64-unknown-linux-gnu/bin/cargo update -Z minimal-versions` (exit status: 101)

@coryan reminds us of #5857 as a way to pre-fetch dependencies with retries. We could adapt this for minimal versions check if we have it list all of the packages+versions it will download. This is doable, but slightly complicated.

I also looked into CARGO_NET_RETRY, which can be used to configure the retry attempts. It seems it is NOT being used, though. https://doc.rust-lang.org/beta/nightly-rustc/cargo/util/network/retry/index.html

We can try to set CARGO_HTTP_MULTIPLEXING=false to trade off download speed for reliability. The bot guesses this would cost us ~30s on the builds.

[dbolduc]

I will wait for a second flake to act.