* guix/profiles.scm (ghc-package-cache-file): New procedure.
(profile-derivation): Add 'ghc-package-cache?' keyword argument. If true
(the default), add the result of 'ghc-package-cache-file' to 'inputs'.
* guix/scripts/package.scm (guix-package)[process-actions]: Pass
#:ghc-package-cache? to 'profile-generation'.
* tests/packages.scm ("--search-paths with pattern"): Likewise.
* tests/profiles.scm ("profile-derivation"): Likewise.