stages/shell.init: add pattern for env var names

Pattern for valid environment variable names as defined in
The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html

Updated tests to match UPPERCASE ONLY var names.

stages/org.osbuild.shell.init

@@ -27,7 +27,10 @@ SCHEMA_2 = r"""
               "additionalProperties": false,
               "required": ["key", "value"],
               "properties": {
-                "key": "string",
+                "key": {
+                  "type": "string",
+                  "pattern": "^[A-Z_][A-Z0-9_]*$"
+                },
                 "value": "string"
               }
             }