Add support for loading PGP keys

Without this, the only way to build PKGBUILDs with signatures is to pass
--skippgpcheck to makepkg.

Also switched to a package with PGP keys to verify.

action.yml

@@ -43,6 +43,14 @@ inputs:
     description: 'Update archlinux-keyring'
     default: 'true'
     required: false
+  pgpkeys:
+    description: 'Comma-separated pgp keys'
+    default: ''
+    required: false
+  pgpkeyserver:
+    description: 'PGP key server'
+    default: 'hkps://keyserver.ubuntu.com'
+    required: false
 
 runs:
   using: 'docker'
@@ -57,3 +65,5 @@ runs:
     - ${{ inputs.namcap }}
     - ${{ inputs.aur }}
     - ${{ inputs.archlinux_keyring }}
+    - ${{ inputs.pgpkeys }}
+    - ${{ inputs.pgpkeyserver }}