bitbake: cooker: return a copy checkPackages

The syntax used for checkPackages implies a copy is returned but it
did not do so. Make it so.

This is fixes universe builds where error messaages were being shown
but there should have only been warnings.

[YOCTO #5222]

(Bitbake rev: 97db2a0792d605f27d434bc6a4acce52857deee1)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/cooker.py

@@ -1192,6 +1192,9 @@ class BBCooker:
 
     def checkPackages(self, pkgs_to_build):
 
+        # Return a copy, don't modify the original
+        pkgs_to_build = pkgs_to_build[:]
+
         if len(pkgs_to_build) == 0:
             raise NothingToBuild