turn path unit into a static

Hi @schaal ,

I see in this [commit][1] that you added the path unit in systemd-crontab-generator.
This works very well.
Does this really need to be generated dynamicaly ? (the result would allways be the same)

Can you review this pull request ?

If @kstep is OK with this, I will also adapt  [systemd-crontab-generator][2] too.

[1] https://github.com/kstep/systemd-crontab-generator/commit/7787ec47ab7b16ee8a79a3e962e360efe612a1a4
[2] https://github.com/kstep/systemd-crontab-generator