Environment variable cleanup can ICE again

[saethlin]

This is a new form of #2020, and the circumstances are perhaps a bit more exotic. I can get ICEs out of rust-minidump and linfa-linalg at the moment, with this:

RUST_BACKTRACE=0 MIRIFLAGS="-Zmiri-panic-on-unsupported -Zmiri-disable-isolation -Zmiri-ignore-leaks" cargo +miri miri test --no-fail-fast -- --test-threads=2

As far as I can tell, all these options are required on rust-minidump, though that is surprising. Output looks like this:

<some more tests>

     Running tests/linux_minidump_writer.rs (target/miri/x86_64-unknown-linux-gnu/debug/deps/linux_minidump_writer-ee88d6f259ec3b82)

running 16 tests
test test_minidump_size_limit ... FAILED
test test_named_threads ... FAILED
test test_named_threads_with_context ... FAILED
test test_sanitized_stacks ... FAILED
test test_sanitized_stacks_with_context ... FAILED
test test_skip_if_requested ... FAILED
thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value: InterpErrorInfo(InterpErrorInfoInner { kind: deallocating while item [SharedReadOnly for <3851536>] is protected by call 1071895, backtrace: None })', src/eval.rs:372:58
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.64.0-nightly (29c5a028b 2022-07-19) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C linker=clang -C incremental -C link-arg=-fuse-ld=mold -Z miri-panic-on-unsupported -Z miri-disable-isolation -Z miri-ignore-leaks

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack

[RalfJung]

Ah yes, if we are unlucky then Stacked Borrows also has things to say about just deallocating memory that other threads are working on...

I think we should probably just not do this cleanup if leaks are ignored anyway.