builder.py: Use TMT_TEST_DATA for storing artifacts for CI

The contents of TMT_PLAN_DATA does not get exposed in the
CI artifacts output UI, whereas TMT_TEST_DATA does.

builder.py

@@ -195,7 +195,7 @@ def tmtCopy(target):
     image = f"{base}.raw.zst"
     manifest = f"{base}.json"
 
-    artifacts = os.getenv("TMT_PLAN_DATA")
+    artifacts = os.getenv("TMT_TEST_DATA")
     if not os.path.exists(artifacts):
         os.mkdir(artifacts)