Replace GHCUP_INSTALL_BASE_PREFIX with GHCUP_HOME

For those who move ~/.ghcup out of their home directories, having .ghcup be a hidden directory is pointless (eg I have GHCUP_INSTALL_BASE_PREFIX=~/.local/opt). It would be preferable to just be able to set GHCUP_HOME=~/.local/opt/ghcup.

A more ambitious version of this would be to XDGify ghcup, but that's much more effort than this itch warrants.

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