UX: `ghcup upgrade` could alert user if upgraded ghcup is shadowed by old ghcup

See #42 (comment 330630).

If one does not read the instructions closely enough, esp. the second half,

Download the binary for your platform at https://downloads.haskell.org/~ghcup/ and place it into your PATH anywhere. Then adjust your PATH in ~/.bashrc (or similar, depending on your shell) like so:

export PATH="$HOME/.cabal/bin:$HOME/.ghcup/bin:$PATH"

one might end up with ghcup upgrade seemingly doing nothing, because the old ghcup is prior to the new .ghcup/bin/ghcup in the PATH.

Being spoiled by stack upgrade and the like, one might expect ghcup upgrade to work failsafe for dummies.
ghcup could check at the end of upgrading whether the new version is actually working, e.g. by evaluating ghcup --version or so. If not, it could alert the user.