This also installs makefile based build system via DESTDIR into a temporary directory and then merges it into the filesystem.
Fixes #364 (closed)
TODO:
mkStaticSubdir :: FilePath -> Path "ghcup"
~/.ghcup/tmp
or so and do moveFile
during file-merge (is that guaranteed to work with XDG spec?)ghc-pkg recache
will have to be re-run)