rustc: Fix permission denied error in 'ar' when lto is enabled #17619
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The reason that 'ar' can fail with permission denied is that when
link-time optimizations are enabled, rustc copies libraries into a
temporary directory, preserving file permissions, and subsequently
modifies them using 'ar'.
The modification can fail because some package managers may install
libraries in system directories as read-only files, which means the
temporary file also becomes read-only when it is copied.
I have fixed this by giving the temporary file's owner read+write
permissions after the copy.
I have also added a regression test for this issue.