Update checks should not warn about updates for uninstalled tool versions #1284
Description
ghcup gives me a warning every time I run it:
[ Warn ] New ghc version available. If you want to install this latest version, run 'ghcup install ghc 9.12.2'
However, I do not have GHC 9.12 installed, and I believe it's not unusual for people not always to be using the latest version of GHC.
Warnings about updates should be given only major versions (x.y) that are actually installed, as they're pointless for tools you're not using.
It may also be worth considering a config file option for skipping update checks, as well as the GHCUP_SKIP_UPDATE_CHECK environment variable, as it's annoying to have to split one's configuration between the config file and the environment.