Add support for overriding variables such as `INSTALL_BASE`

I notice global vars such as

INSTALL_BASE="$HOME/.ghcup"
GHC_LOCATION="$INSTALL_BASE/ghc"
BIN_LOCATION="$INSTALL_BASE/bin"

which would be desirable to allow to be customized in a way that survives self-update; I can think of two common options, either

1. have vars such as INSTALL_BASE default to set the value of e.g. a GHCUP_INSTALL_BASE env-var if it's set and non-empty (useful for export GHCUP_INSTALL_BASE=... in your .profile script), or
2. simply add a couple long-opts, --install-base=, --ghc-location=... etc (this however requires using an alias in your .profile scripts if it's supposed to be a persistent setting)