Garbage collecting cache fails when using XDG directories

[tfausak]

With GHCUP_USE_XDG_DIRS=1 set, running ghcup gc --cache fails for me:

vscode ➜ /workspaces/scratch $ env | grep -i ghcup
GHCUP_USE_XDG_DIRS=1

vscode ➜ /workspaces/scratch $ ghcup --version
The GHCup Haskell installer, version v0.1.19.2-8-gc20dece

vscode ➜ /workspaces/scratch $ ghcup gc --cache
[ Info  ] downloading: https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-0.0.7.yaml as file /home/vscode/.cache/ghcup/ghcup-0.0.7.yaml
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  232k  100  232k    0     0  2347k      0 --:--:-- --:--:-- --:--:-- 2347k
[ Warn  ] New ghc version available. If you want to install this latest version, run 'ghcup install ghc 9.6.1'
ghcup: /home/vscode/.cache/ghcup/tmp: removeLink: inappropriate type (Is a directory)

vscode ➜ /workspaces/scratch $ ls -A -l ~/.cache/ghcup/tmp
total 0

[hasufell]

$ ls ~/.cache/ghcup/
 db   ghcup-0.0.6.yaml   ghcup-0.0.6.yaml.etags   ghcup-0.0.7.yaml   ghcup-0.0.7.yaml.etags   logs   tmp

This doesn't work, because ghcup gc --cache expects the cache dir to only have files. It will refuse to delete directories (because it's unsafe).

There are two things we need to do:

1. we should use a subdir ~/.cache/ghcup/cache
2. ~/.cache/ghcup/db should in fact live in ~/.local/share/ghcup/db