When installing plugin permissions, we try to set the permissions on all
installed files ourselves because a umask from the user could violate
everything needed to get the permissions right. Sadly, directories were
not handled correctly at all and so we were still left with broken
installations with umasks like 0077. This commit fixes this issue, adds
a thorough unit test for the situation, and most importantly, adds a
test that sets the umask before installing the plugin.
Relates #24527
3e485c2ca5