this makes it easier for one to manually administer freshrss.
for example, i can import OPML from the CLI like:
```
$ nix build .#freshrss
$ freshrss FRESHRSS_DATA_PATH=/var/lib/freshrss ./result/cli/import-for-user.php --user admin --file my-opml.opml
```
whereas previously i would have needed to include
`environment.systemPackages = [ php ];` in my system for that to work.
