thelounge: re-apply 'write out default path'

[Mrmaxmeier]

Re-apply
df2f8d9#diff-55e70513e1cc0032a186d6b7b367679aR128-R130

(cc #70318)

It seems like this got lost in #89184:

6602f87#diff-55e70513e1cc0032a186d6b7b367679aL126-L131

6602f87#diff-f7e3726c9a96ebbbffbb4e2ac12ba6d7R126-R128

With nice inline previews:

Before:

nixpkgs/pkgs/development/node-packages/default-v10.nix

Lines 126 to 131 in 33f1160

	thelounge = nodePackages.thelounge.override {
	buildInputs = [ nodePackages.node-pre-gyp ];
	postInstall = ''
	echo /var/lib/thelounge > $out/lib/node_modules/thelounge/.thelounge_home
	'';
	};

After:

nixpkgs/pkgs/development/node-packages/default-v12.nix

Lines 126 to 128 in 6602f87

	thelounge = nodePackages.thelounge.override {
	buildInputs = [ nodePackages.node-pre-gyp ];
	};

Motivation for this change

My thelounge-instance died after upgrading to the latest nixos-unstable.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.