bug#55270: unpack phase randomly changes the cwd

[Attila Lendvai]

at the end of the unpack phase, the working directory is changed to a random 
directory.

https://git.savannah.gnu.org/cgit/guix.git/tree/guix/build/gnu-build-system.scm#n178

this is fine *when* the archive contains a single dir... but this DWIM'ness has 
just burned 15 mintues of my life, and i recommend removing it.

if it is to stay, then at least it should be patched that it only happens when 
the dir after extraction only contains a single subdir, and no files otherwise.

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• attila lendvai
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