Enable gpg verification

[lazka]

• Enable keyserver-options auto-key-retrieve in ~/.gnupg/gpg.conf
• Maybe use the Ubuntu keysever because is is less flaky
• Don't pass --skippgpcheck when building

[lazka]

It's now enabled in CI for both package repos:

• https://github.com/msys2/MSYS2-packages/commit/1b13365a48166fedb0e64fb48e54274230670df0
• https://github.com/msys2/MINGW-packages/commit/cb219f9ff9ea34159649ce5593f77b1f47e9c5d4

which means if it passes there it will also pass here once enabled.