nix: fix nixosModule and build (#1979)

* lnbits/settings: get LNBITS_COMMIT from envvar if available

There is no other way of setting the git commit at runtime, for build systems like Nix that allow you to hermetically define envvars derived from the source that will be available reproducibly at runtime

* nix: update potree2nix and nixosModule

This is a general refactor that gets everything building and passing the vmTest again

* Update nix/modules/lnbits-service.nix

---------

Co-authored-by: Pavol Rusnak <pavol@rusnak.io>

nix/tests/nixos-module/default.nix

@@ -1,5 +1,6 @@
 { pkgs, makeTest, inputs }:
 makeTest {
+  name = "lnbits-nixos-module";
   nodes = {
     client = { config, pkgs, ... }: {
       environment.systemPackages = [ pkgs.curl ];