Really disabled building for epel8

I tried to disable this in a9639e680b, but
it was still built.  I believe el8 is the correct platform name to
filter out to prevent stream expansion from building in epel8.  See
rhbz#2042653 for more information.

nextcloud.yaml

@@ -15,9 +15,9 @@ data:
             - MIT
     dependencies:
       - buildrequires:
-            platform: [-eln, -epel8]
+            platform: [-eln, -el8]
         requires:
-            platform: [-eln, -epel8]
+            platform: [-eln, -el8]
     references:
         documentation: https://docs.nextcloud.com/server/latest/admin_manual/
         tracker: https://github.com/nextcloud/server/issues