ci/github-script: move from ci/labels

This just moves things around to use less specific naming - `labels` is
only *one* script that can potentially be run locally while still being
written in github-script. Later, we can add more.

ci/github-script/shell.nix

@@ -0,0 +1,23 @@
+{
+  system ? builtins.currentSystem,
+  pkgs ? (import ../. { inherit system; }).pkgs,
+}:
+
+pkgs.callPackage (
+  {
+    mkShell,
+    importNpmLock,
+    nodejs,
+  }:
+  mkShell {
+    packages = [
+      importNpmLock.hooks.linkNodeModulesHook
+      nodejs
+    ];
+
+    npmDeps = importNpmLock.buildNodeModules {
+      npmRoot = ./.;
+      inherit nodejs;
+    };
+  }
+) { }