buildBazelPackage: set $USER environment variable (#74538)

tensorflow assumes $USER to be set to something, otherwise it complains
like this:

```
FATAL: $USER is not set, and unable to look up name of current user: (error: 0): Success
Traceback (most recent call last):
  File "./configure.py", line 1602, in <module>
    main()
  File "./configure.py", line 1399, in main
    _TF_MAX_BAZEL_VERSION)
  File "./configure.py", line 478, in check_bazel_version
    ['bazel', '--batch', '--bazelrc=/dev/null', 'version'])
  File "./configure.py", line 156, in run_shell
    output = subprocess.check_output(cmd)
  File "/nix/store/drr8qcgiccfc5by09r5zc30flgwh1mbx-python3-3.7.5/lib/python3.7/subprocess.py", line 411, in check_output
    **kwargs).stdout
  File "/nix/store/drr8qcgiccfc5by09r5zc30flgwh1mbx-python3-3.7.5/lib/python3.7/subprocess.py", line 512, in run
    output=stdout, stderr=stderr)
```

Spotted while changing the hash of its fixed-output derivation on
purpose.

We could also set this in the tensorflow-specific part, but very likely,
other programs will fail as well.