Always log the username and hostname in a processing job

[JamesWrigley]

This can be helpful when debugging.

(I'm roguishly gonna go ahead and merge this because it'd be quite useful ATM)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.32%. Comparing base (e2677b0) to head (1c633c2). Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
damnit/backend/extract_data.py	20.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #348 +/- ## ========================================== - Coverage 75.35% 75.32% -0.03% ========================================== Files 32 32 Lines 4974 4977 +3 ========================================== + Hits 3748 3749 +1 - Misses 1226 1228 +2 ```

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[takluyver]

LGTM, thanks!