Revert "Add Nuget to Mono package."

This reverts commit cded6685a37ec01bc5b6c85dc70368c7a6c01953.
Purge prefix path before starting a build.
Previously, the prefix path was left alone, and the build-root was purged if MONO_VERSION changed.
Now the prefix path is *always* removed, as it can contain a partially successful build that is invalid despite MONO_VERSION remaining the same.
This also means that we can't detect MONO_VERSION changes between bockbuild runs on the same host, but purging build-root does little to ensure build consistency compared to actually purging the install path.
Always install built packages.
At this point, install_success_file ('.install') is ignored, as we are always re-installing.
This should be much safer than previous logic, which would try to forego installing. This made lane packaging on wrench unreliable and hard to debug.
Bockbuild now only caches checkouts and builds, but does not "resume" from incomplete (and possibly problematic) packaging material.
Build fontconfig with --with-cache-dir.

This enables fontconfig to correctly fallback to caching to ~/Library/Caches if mono
is not installed. This affects XamMac apps as fontconfig would rebuild its cache every single time.
Build tar with --enable-nls=no, without this, it seems to crash when invoking gzip if LC_CTYPE/LANG are not set.
Revert "Build fontconfig with --with-cache-dir."
The commit breaks the build, as fontconfig's configure scripts don't seem to be able to handle '~' in the '--with-cache-dir' arguments. Alternatively using ${HOME} instead actually works, but a) Bockbuild's own macro expansion rules try to expand ${HOME} and b) cache-dir with ${HOME} seems to be expanded at build-time which I do not think was the idea for this change. Will discuss with @duncanmak and @kumpera.

This reverts commit 0aac9b0881029bff125e11ae4bc7e4c223f828da.