misc: Updated resources bucket url to version 20

This will fix this v20.0.0.0 of gem5 to the v20 gem5 resources bucket
subdirectory.

Change-Id: I0c1f1a2c28c18e684bcb009d1650f9e9d2950a93
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28708
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

ext/testlib/config.py

@@ -220,7 +220,7 @@ def define_defaults(defaults):
                                                       os.pardir))
     defaults.result_path = os.path.join(os.getcwd(), '.testing-results')
     defaults.list_only_failed = False
-    defaults.resource_url = 'http://dist.gem5.org/dist/develop'
+    defaults.resource_url = 'http://dist.gem5.org/dist/v20'
 
 def define_constants(constants):
     '''