Prepare 21.2 release branch

Author: gilles-duboscq

ci.jsonnet

@@ -10,7 +10,7 @@
 # https://github.com/google/jsonnet/releases and compiled.
 
 # CONFIGURATION
-local overlay = "a8ffba29647185aa9d6c1191be546ef5d9f18708";
+local overlay = "429f4a03748bab84b7953c634f2b93136c4f649d";
 
 # For debugging: generated builds will be restricted to those listed in
 # the array. No restriction is applied when it is empty.
@@ -44,6 +44,7 @@ local part_definitions = {
       },
 
       packages+: {
+        "mx": "5.302.2",
         "pip:ninja_syntax": "==1.7.2",  # Required by NFI and mx
       },