Get cabal from the default channel

This means we don't have to include cabal metadata in our generated metadata.

An alternative would be to use "add-release-channel" command but it seems safer to actually override the metadata for installing ghc so we are pretty confident we are testing the right GHC.

See: https://www.haskell.org/ghcup/guide/#pre-release-channels

.gitlab/install-bindist.sh

@@ -17,7 +17,7 @@ ghcup --version
 which ghcup | grep foobarbaz
 
 ghcup --metadata-caching=0 -v --url-source=$METADATA_URI install ghc --set $VERSION
-ghcup --metadata-caching=0 -v --url-source=$METADATA_URI install cabal
+ghcup --metadata-caching=0 -v install cabal
 
 mkdir $runner_temp/install-bindist-ci
 cd $runner_temp/install-bindist-ci