zip: set PWD consistently for commands in subdirectories
zip: set PWD consistently for commands in subdirectories

Some platforms (notably macOS) use symlinks for /tmp, so if we fail to
set PWD appropriately, any resulting errors may refer to the wrong
directory, making those errors more confusing.

Change-Id: Ibc899d7aca138d9e016171e34e7e4aab4895a4df
Reviewed-on: https://go-review.googlesource.com/c/mod/+/375914
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Auto-Submit: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/mod,golang/mod