synopsis: Use default install directory if not specified
packages: cabal-install
prs: #6624
issues: #5973

description: {

Cabal 3.0.0.0 added the --installdir option to specify the location that
binaries should be installed in. Running a cabal user-config update would
populate the config file with the default value, but the cabal install program
would error if it wasn't set. This change uses the default value that would be
written to the config if its unset, and outputs a warning.

}
