nix: generate application package build script

This takes some metadata, sandbox options, a launch script and a list of home-manager modules. The result needs to be executed in an environment with nix daemon access, and it produces the final package file.

Signed-off-by: Ophestra <cat@gensokyo.uk>

.gitignore

@@ -4,6 +4,7 @@
 *.dll
 *.so
 *.dylib
+*.pkg
 /fortify
 
 # Test binary, built with `go test -c`