This commit, after reverting #55359, applies a different fix for #46775
while also fixing #55775. The basic idea was to go back to pre-#55359
libstd, and then fix#46775 in a way that doesn't expose #55775.
The issue described in #46775 boils down to two problems:
* First, the global environment is reset during `exec` but, but if the
`exec` call fails then the global environment was a dangling pointer
into free'd memory as the block of memory was deallocated when
`Command` is dropped. This is fixed in this commit by installing a
`Drop` stack object which ensures that the `environ` pointer is
preserved on a failing `exec`.
* Second, the global environment was accessed in an unsynchronized
fashion during `exec`. This was fixed by ensuring that the
Rust-specific environment lock is acquired for these system-level
operations.
Thanks to Alex Gaynor for pioneering the solution here!
Closes#55775
Co-authored-by: Alex Gaynor <alex.gaynor@gmail.com>
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