rhel84: use en_US.UTF-8 as default for LANG

Use en_US.UTF-8 as default for LANG, which is what previously was
used and is also needed to properly work on non-us/latin setups[1].

In the customization tests, use a different value than the default
one to check that the customization does in fact work.

[1] http://git.app.eng.bos.redhat.com/git/spin-kickstarts.git/tree/rhel8/rhel-8.2-kvm-x86_64.ks#n4

Co-authored-by: Achilleas Koutsou <achilleas@koutsou.net>

test/data/manifests/fedora_33-x86_64-qcow2-customize.json

@@ -67,7 +67,7 @@
         },
         "locale": {
           "languages": [
-            "en_US"
+            "el_CY.UTF-8"
           ],
           "keyboard": "dvorak"
         },
@@ -3483,7 +3483,7 @@
         {
           "name": "org.osbuild.locale",
           "options": {
-            "language": "en_US"
+            "language": "el_CY.UTF-8"
           }
         },
         {