makebuildserver: cache defaults to ~/.cache/fdroidserver

makebuildserver: cache defaults to ~/.cache/fdroidserver

This also provides a config option to override that default.  ~/.cache is
a standard location on GNU/Linux machines for cached content.  It is also
good to have the cache outside of the git repo in case `git clean -fdx` is
run, which would delete all files in the directory that are not part of the
git repo, including buildserver/cache/