Smoother upgrade path from ghcup 0.0.7

What I did:

$ ghcup --help
ghcup 0.0.7
...
$ ghcup upgrade

Expected result: it upgrades ghcup.

Actual result: it replaced ~/.ghcup/bin/ghcup with garbage, presumably due to not handling the redirect correctly:

$ cat ~/.ghcup/bin/ghcup
<a href="/haskell/ghcup/master/ghcup">Moved Permanently</a>