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.
a905fcb40d