Enable RUST_BACKTRACE=1 in tests. (#11670)
Enable RUST_BACKTRACE=1 in tests. (#11670)

### Problem

`RUST_BACKTRACE` is not passed through to hermetic tests, but we'd like it set in all cases there.

### Solution

Set `RUST_BACKTRACE=1` for tests.

[ci skip-rust]
[ci skip-build-wheels]
pantsbuild/pants,pantsbuild/pants,benjyw/pants,jsirois/pants,jsirois/pants,benjyw/pants,pantsbuild/pants,jsirois/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants