Allowlist RBE_num_retries_if_mismatched variable

This variable tells reproxy the number of times an action that
mismatches between local and remote-executions should be run remotely to
determine if the action is inherently non-deterministic.
An action is considered non-deterministic if the number of unique output
digests we get from the remote-run is > 1.

Bug: b/178842439
Change-Id: I0d50eed29b801be67a7ede9b25ce7f4535980f05
diff --git a/ui/build/ninja.go b/ui/build/ninja.go
index 893fd6d..5961c45 100644
--- a/ui/build/ninja.go
+++ b/ui/build/ninja.go
@@ -145,6 +145,7 @@
 			"RBE_exec_strategy",
 			"RBE_invocation_id",
 			"RBE_log_dir",
+			"RBE_num_retries_if_mismatched",
 			"RBE_platform",
 			"RBE_remote_accept_cache",
 			"RBE_remote_update_cache",