Windows is a ridiculous operating system designed by toddlers, and so
requires us to close all file handles in the `tmp/xxxxxxx` cache dir
before renaming it into `o/xxxxxxx`. We have a hack in place to handle
this for the main output file, but the MachO linker also outputs a file
with debug symbols, and we weren't closing it! This led to a fuckton of
CI failures when we enabled `.whole` cache mode by default for
self-hosted backends.

thanks jacob for figuring this out while i sat there