builder: rename gpg_key field to gpgkey for repos

We decided to do this late change to Cloud API in order to keep the field name
same for all the APIs:

osbuild/osbuild-composer#2479

I strongly believe that gpgkeys are not used in any of the current koji-osbuild
deployments yet so this change should be safe.

plugins/builder/osbuild.py

@@ -103,7 +103,7 @@ def as_dict(self, arch: str = ""):
             "rhsm": self.rhsm
         }
         if self.gpgkey:
-            res["gpg_key"] = self.gpgkey
+            res["gpgkey"] = self.gpgkey
             res["check_gpg"] = True
         if self.package_sets:
             res["package_sets"] = self.package_sets