Hello Guix!

I just realised the gpodder package was not deterministic because of
'POT-Creation-Date' entries in the .po files. I forgot to check for
determinism before sending that patch, whoops! :-)

Tested with:

```
./pre-inst-env guix build --no-grafts --check --rounds=2 gpodder
(...)
/gnu/store/15l476abksby4awsnvckm3ai59fgxb9x-gpodder-3.10.7
```

Which would give us:
```
guix build: error: derivation `/gnu/store/k0my78v8n5r9xdxcip3q7dm6xzb80-gpodder-3.10.7.drv' may not be deterministic: output `/gnu/store/fmaplc6y2dv4ys3n4fcpziaw83p3smf4-gpodder-3.10.7' differs
```

Thanks!
Pierre