Perl installs a couple of config files that need to be munged so that
they don't refer to the spack compiler. These files are installed
read-only. Behind the scenes 'filter_file' moves its file to a safe
place, and tries to create a working file that is both O_WRONLY and
has the perms of the original file. On an NFSv4 filesystem, the
combination of 'r--r--r--' and O_WRONLY throws a permissions error.
This commit adds a simple context manager that temporarily makes the
files writable.
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