ghc-pkg: don't expand ${name}-style env vars by default

For shell-based build systems the feature is still available as:
  ghc-pkg register --expand-env-vars

Historically, ghc-pkg allowed environment variables to appear in the
input files for ghc-pkg register. They are not stored in the package
database but are expanded upon registration. This feature helped for
build systems based on makefiles and shell scripts. These days the
vast majority of such files are generated by Cabal and we don't want
any ${name} strings (e.g. perhaps in a package description) getting
accidentally interpreted as an environment variable.