Rollup merge of #82949 - the8472:forget-envlock-on-fork, r=joshtriplett

Rollup merge of #82949 - the8472:forget-envlock-on-fork, r=joshtriplett

Do not attempt to unlock envlock in child process after a fork.

This implements the first two points from https://github.com/rust-lang/rust/issues/64718#issuecomment-793030479

This is a breaking change for cases where the environment is accessed in a Command::pre_exec closure. Except for single-threaded programs these uses were not correct anyway since they aren't async-signal safe.

Note that we had a ui test that explicitly tried `env::set_var` in `pre_exec`. As expected it failed with these changes when I tested locally.