Drop accidental write-attributes request
The new filesystem code accidentally asks for write attributes
permissions when doing read-only access.
I believe this is what's causing the GHC 8.6.1 tarballs to fail
when installed to a privileged location.
I haven't been able to reproduce the issue yet, but this permission
bit is wrong anyway.
Test Plan: I'm still trying to workout how to test that this works,
changing the permissions on the folder doesn't seem to reproduce
the error on a tarball I made from before the change.
Reviewers: bgamari, tdammers
Reviewed By: bgamari
Subscribers: tdammers, monoidal, rwbarton, carter
GHC Trac Issues: Trac #15667
Differential Revision: https://phabricator.haskell.org/D5177