Add runHydra shell

runHyda automatically starts hydra and postgres:

```
$ nix-shell -A runHydra
```

The shell receives hydra from the working copy as buildInput.
Running hydra, queue-runner, evaluator and postgres is managed
by foreman (https://github.com/ddollar/foreman) and configured
in `Procfile`.

flake.nix

@@ -104,13 +104,16 @@
             [ makeWrapper autoconf automake libtool unzip nukeReferences pkgconfig libpqxx
               gitAndTools.topGit mercurial darcs subversion bazaar openssl bzip2 libxslt
               perlDeps perl final.nix
-              postgresql95 # for running the tests
               boost
               (if lib.versionAtLeast lib.version "20.03pre"
                then nlohmann_json
                else nlohmann_json.override { multipleHeaders = true; })
             ];
 
+          checkInputs = [
+            postgresql95
+          ];
+
           hydraPath = lib.makeBinPath (
             [ subversion openssh final.nix coreutils findutils pixz
               gzip bzip2 lzma gnutar unzip git gitAndTools.topGit mercurial darcs gnused bazaar
@@ -290,6 +293,8 @@
         nixpkgs.overlays = [ self.overlay nix.overlay ];
       };
 
+      runHydra = pkgs.callPackage ./run-hydra.nix {};
+
       nixosModules.hydraTest = {
         imports = [ self.nixosModules.hydra ];