Summary:
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: #15667

Differential Revision: https://phabricator.haskell.org/D5177

(cherry picked from commit deceb21b)