[windows] ghcup saves executable without .exe extension on upgrade and doesn't detect itself if it is a release candidate

Hi, trying the recent added support i've found a strange behavior that i guess is only in windows:

• when upgrading itself ghcup warns that it is not in PATH, when it is:

PS D:\hls> ghcup upgrade
[ Info  ] Upgrading GHCup...
[ Info  ] downloading: https://downloads.haskell.org/~ghcup/0.1.15.1/x86_64-mingw64-ghcup-0.1.15.1.exe
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 27.1M  100 27.1M    0     0  4633k      0  0:00:06  0:00:06 --:--:-- 5069k
[ Info  ] verifying digest of: ghcup.exe
[ Warn  ] "ghcup" is not in PATH! You have to add it in order to use ghcup.
[ Info  ] Successfully upgraded GHCup to version 0.1.15.1
PS D:\hls> ghcup --version
The GHCup Haskell installer, version v0.1.15-rc2-15-gecfed1a
PS D:\hls> (gcm ghcup).Source
D:\ghcup\bin\ghcup.exe

• when running ghcup list ghcup itself is not shown as installed:

PS D:\hls> ghcup list
[ Warn  ] New GHCup version available: 0.1.15.1. To upgrade, run 'ghcup upgrade'
[ Warn  ] New GHC version available: 9.0.1. To upgrade, run 'ghcup install ghc 9.0.1'
   Tool  Version        Tags                      Notes
X  ghc   7.10.3         base-4.8.2.0
X  ghc   8.0.2          base-4.9.1.0
X  ghc   8.2.2          base-4.10.1.0
X  ghc   8.4.1          base-4.11.0.0
X  ghc   8.4.2          base-4.11.1.0
X  ghc   8.4.3          base-4.11.1.0
X  ghc   8.4.4          base-4.11.1.0
X  ghc   8.6.1          base-4.12.0.0
X  ghc   8.6.2          base-4.12.0.0
X  ghc   8.6.3          base-4.12.0.0
X  ghc   8.6.4          base-4.12.0.0
X  ghc   8.6.5          base-4.12.0.0
X  ghc   8.8.1          base-4.13.0.0
X  ghc   8.8.2          base-4.13.0.0
X  ghc   8.8.3          base-4.13.0.0
X  ghc   8.8.4          base-4.13.0.0
X  ghc   8.10.1         base-4.14.0.0
X  ghc   8.10.2         base-4.14.1.0
X  ghc   8.10.3         base-4.14.1.0
X  ghc   8.10.4         base-4.14.1.0
IS ghc   8.10.5         recommended,base-4.14.2.0
X  ghc   9.0.1          latest,base-4.15.0.0
X  ghc   9.2.0.20210422 prerelease,base-4.16.0.0
X  cabal 2.4.1.0
X  cabal 3.0.0.0
X  cabal 3.2.0.0
IS cabal 3.4.0.0        latest,recommended
X  hls   1.1.0          latest,recommended
X  stack 2.5.1
IS stack 2.7.1          latest,recommended
X  ghcup 0.1.15.1       latest,recommended