Set --remote_cache to --remote_executor if it is empty.

[coeuvre]

Fixes #11913

[ulfjack]

I'm not sure that this is the right fix. Conceptually, combining --remote_cache and --remote_executor would seem to imply that you're using different cache and executor, which is generally probably not what users want. I think it would make more sense to me to disallow having both set at the same time rather than doing this. (Now, that doesn't answer the question of how to fix the linked issue.)

[coeuvre]

Pointing --remote_cache and --remote_executor to different endpoints is allowed currently and I'm not sure whether users are using this feature or not. This PR tries to fix the linked issue without breaking current behavior. How about create another issue describing the problem you mentioned?

[coeuvre]

It seems that users are depended on combining --remote_cache and --remote_executor in some cases. See #11901 and some discussion in bazelbuild/remote-apis#61.