[conda] Add QT_QPA_PLATFORM to test environment variable

This will avoid having to set that at the all the projects level
that run builds since we want to use it in any case and whatever
the runner base OS is.

conda/meta.yaml

@@ -46,6 +46,9 @@ test:
   imports:
     - {{ name }}
 
+  script_env:
+    - QT_QPA_PLATFORM=offscreen
+
   commands:
     - beat editor --help
     - beat editor start --help