mirror: make _run_wget() return to the dir it started in
mirror: make _run_wget() return to the dir it started in

This kind of function should not change the working environment, especially
since so much of the fdroidserver code assumes the current working directory
is the root of an fdroid repo.

It is used in fdroidserver/mirror.py with absolute paths always, so it
shouldn't change any existing use.  I found this issue by using it in a
plugin.

fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver