zeroinstall: changed gnupg to Requirement.

https://github.com/Homebrew/homebrew/pull/28771/
zeroinstall: moved head stuff to a block.

https://github.com/Homebrew/homebrew/pull/28771/
zeroinstall: removed PKG_CONFIG_PATH fiddling.

https://github.com/Homebrew/homebrew/pull/28771/