Bazel fails to start up if $USER is not set plus no valid UID can be retrieved

### Description of the bug:

Bazel tries to [get user name to figure out output_user_root](https://github.com/bazelbuild/bazel/blob/756be22f6445d6052a0a9cce66fdb7f7b3a8f300/src/main/cpp/startup_options.cc#L124) even if `--output_user_root` is passed in. However, `GetUserName` is a fallible call in an environment where $USER is unset, and `getuid` or `getpwuid` don't work properly. Such situation is possible in isolated environments like processed launched by nsjail.

The only option seems to be setting $USER, which is actually unused if `--output_user_root` is specified. I'd expect setting `--output_user_root` to be good enough for getting around this.

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

```
# install nsjail, then
./nsjail -Mo --chroot / --user 99999 --group 99999 -- /bin/sh -i
...
sh-5.1$ bazel
FATAL: $USER is not set, and unable to look up name of current user: (error: 0): Success
```

### Which operating system are you running Bazel on?

Linux 5.18.16-1rodete1-amd64

### What is the output of `bazel info release`?

release 5.3.1

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start in src/main/cpp/startup_options.cc at the GetUserName call linked in the report, then reproduce the failure in the provided nsjail environment with $USER unset and no valid UID lookup. Check that supplying --output_user_root avoids the user-name lookup and that Bazel starts successfully in this case.

Written by the indexing model from the issue text.