RunCommand: Add a WIP execution of dynamic commands

This in-progress feature will run a dynamically generated set of
buildFinished hooks, which must be nested under the `runCommandHook.*`
attribute set. This implementation is not very good, with some to-dos:

1. Only run if the build succeeded
2. Verify the output is named $out and that it is an executable file
   (or a symlink to a file)
3. Require the jobset itself have a flag enabling the feature, since
   this feature can be a bit dangerous if various people of different
   trust levels can create the jobs.

t/jobs/runcommand-dynamic.nix

@@ -0,0 +1,27 @@
+with import ./config.nix;
+{
+  runCommandHook.example = mkDerivation
+    {
+      name = "my-build-product";
+      builder = "/bin/sh";
+      outputs = [ "out" "bin" ];
+      args = [
+        (
+          builtins.toFile "builder.sh" ''
+            #! /bin/sh
+
+            echo "$PATH"
+
+            mkdir $bin
+            echo "foo" > $bin/bar
+
+            metrics=$out/nix-support/hydra-metrics
+            mkdir -p "$(dirname "$metrics")"
+            echo "lineCoverage 18 %" >> "$metrics"
+            echo "maxResident 27 KiB" >> "$metrics"
+          ''
+        )
+      ];
+    };
+
+}