Commit f086bc9564 (Maintain ownership when rewriting universal variables
file, 2015-09-26) fixed an issue where "sudo -E fish" would create root-owned
~/.config/fish/fish_variables that break the users's shell.
A simlar issue exists for files in ~/.cache/fish; fix that.
Note that ~/.cache/fish is currently created on first run when we generate
completions from manpages.
See the issue described in #11292
We have a lot of completions that look like
```fish
pip completion --fish 2>/dev/null | source
```
That's *fine*, upstream gives us some support.
However, the scripts they provide change very rarely, usually not even
every release, and so running them again for every shell is extremely
wasteful.
In particular the python tools are very slow, `pip completion --fish`
takes about 180ms on my system with a hot cache, which is quite
noticeable.
So what we do is we run them once, store them in a file in our cache
directory, and then serve from that.
We store the mtime of the command we ran, and compare against that for
future runs. If the mtime differs - so if the command was up or
downgraded, we run it again.
