`ghcup upgrade` fails with `.ghcup/bin/ghcup: openFd: does not exist`

1. Downloaded ghcup (mac) from https://downloads.haskell.org/~ghcup/.
2. Placed it in dir.
3. Symbolically linked it from my-bin-dir which is in `PATH.
4. Tried to upgrade as prompted.

$ ghcup upgrade
[ Info  ] Upgrading GHCup...
[ Info  ] downloading: https://downloads.haskell.org/~ghcup/0.1.12/x86_64-apple-darwin-ghcup-0.1.12
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 8796k  100 8796k    0     0   228k      0  0:00:38  0:00:38 --:--:--  231k
[ Info  ] verifying digest of: ghcup
[ Error ] CopyError "/Users/abel/.ghcup/bin/ghcup: openFd: does not exist (No such file or directory)"

It seems to rely that .ghcup/bin always exists, but did not care to created it. After manually adding this subdirectory of .ghcup, upgrade worked.

Proposed fix of upgrade procedure: Create bin if it does not exist yet.