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