Nginx module should allow config passed in as path

[copumpkin]

In general, it's much more pleasant to go from a string to a path than the other way around, since builtins.readFile then forces import-from-derivation and other unpleasantness. The nginx module today unfortunately expects config and appendConfig to be strings, which means that if my config file comes from a derivation (e.g., pkgs.substituteAll) tha forces me to build during evaluation. It doesn't seem terribly hard to support the other use case by deferring the writeText a bit more.

[stale]

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[tomodachi94]

cc @RaitoBezarius @fpletz, who are maintainers on the package