Remote execution builds are failed on grpc status code UNAVAILABLE

Trying to use remote execution to build, I often get these errors (often to the point I can't get a full build done, especially that there seems to be a bug in caching elements)

```
[--:--:--] START freedesktop-sdk.bst:components/pixman.bst: Uploading input root
[00:00:09] SUCCESS freedesktop-sdk.bst:components/pixman.bst: Uploading input root
[--:--:--] START freedesktop-sdk.bst:components/pixman.bst: Waiting for the remote build to complete
[00:01:00] FAILURE freedesktop-sdk.bst:components/pixman.bst: Waiting for the remote build to complete
[00:01:09] FAILURE freedesktop-sdk.bst:components/pixman.bst: Running commands
[00:01:12] FAILURE [87a4afe9] freedesktop-sdk.bst:components/pixman.bst: Failed contacting remote execution server at http://ab84137533a384b2ea288af9529636c2-1891086637.us-east-1.elb.amazonaws.com:8980.UNAVAILABLE/: Socket closed
```

Looking at the grpc documentation, the UNAVAILABLE status code should be retried.

The issue names no source files or tests; start by locating the remote-execution client and its gRPC status handling. Reproduce the reported UNAVAILABLE failure, then verify that transient unavailable responses are retried and a remote build can complete without the reported failure.

Written by the indexing model from the issue text.