brew: Install `gnupg` for Mac only to save setup time

gnupg is added by !2739 (merged) in response to #1195 (closed), to install specific yarn version with asdf-yarn plugin.

gpg should be preinstalled on Linux, and installing it additionally with brew takes 3m+

$ time brew install gpg
...
real    3m33.144s
user    2m16.817s
sys     1m24.092s