CI tests using rust nightly are failing

[alamb]

Describe the bug
The arrow CI tests that rely on nightly are crashing with some sort of rustc internal compiler error

for example: https://github.com/apache/arrow-rs/runs/4468916693?check_suite_focus=true

thread 'rustc' panicked at 'assertion failed: sentinel == STR_SENTINEL', /rustc/e6b883c74f49f32cb5d1cbad3457f2b8805a4a38/compiler/rustc_serialize/src/opaque.rs:669:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Also on master: https://github.com/apache/arrow-rs/runs/4474503693?check_suite_focus=true

As @b41sh pointed out #779 (comment), it seems to be a bug in rust: rust-lang/rust#91663

To Reproduce
make a PR with no code changes

Expected behavior
CI should pass

Additional context
Add any other context about the problem here.

[alamb]

I plan to pin nightly to some version that was working until the upstream bug is fixed

[alamb]

Upon reading rust-lang/rust#91663 more carefully I first plan to try and force a clean cache first