dev-cmd/test: set `RUST_BACKTRACE` when retrying
dev-cmd/test: set `RUST_BACKTRACE` when retrying

Occasionally, a new version of Rust will cause failures in dependents.
See, for example, Homebrew/homebrew-core#107818.

This change will allow us to get a fuller backtrace in CI, which will
also make it easier to submit bug reports upstream.

Without this change, recovering a full backtrace requires installing the
new version of Rust and rebuilding the failing formula from source. Both
these steps can be skipped if we set `RUST_BACKTRACE` when retrying a
failing test.

MikeMcQuaid/brew,Homebrew/brew,JCount/brew,nandub/brew,JCount/brew,Homebrew/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,JCount/brew,JCount/brew,MikeMcQuaid/brew,Homebrew/brew,nandub/brew,nandub/brew,nandub/brew,Homebrew/brew