feat: add support for nix store paths in variables

by exporting them at runtime and removing them from the pipeline
definition itself
closes #3

flake.nix

@@ -42,9 +42,14 @@
               when = "delayed";
               start_in = "1 hour";
               deps = [pkgs.hello pkgs.curl];
+              variables = {
+                TEST = "test";
+                TEST_WITH_DERIVATION = "${pkgs.hello}/test";
+              };
               script = [
                 "hello"
                 "curl google.de"
+                "echo $TEST $TEST_WITH_DERIVATION"
               ];
             };
             "test-non-nix" = {