Consider conditions to not include libstd in archives

[sunshowers]

With cargo-nextest 0.9.71, we now include libstd unconditionally in archives. This is done for two reasons:

• proc-macro tests
• -C prefer-dynamic

As of Rust 1.78, this makes archives:

• around 4MB bigger without cross-compilation
• around 8MB bigger with cross-compilation

This doesn't seem to be hugely relevant especially compared to the size of Rust test binaries themselves, but if this is a problem for you, please post in this issue.

I don't plan to work on fixing this, but one way we could address this is by adding config knobs. If this impacts you, thoughts on how to configure this, as well as concrete contributions in the form of PRs, would be welcome.