hydra-module: don't use createHome = true; to create Hydra's base-dir

In NixOS, the user generation script was changed to set the permissions `0700`
to a home-directory that's specified in the `users.users`-submodule with
`createHome` being set to `true`[1].

However, the home-directory of `hydra` is also the base directory of other services using
other users (e.g. `hydra-queue-runner`). With permissions being `0700`, processes with
such a user cannot traverse into `/var/lib/hydra` and thus not into subdirectories.

I guess that this issue was kind of hidden because `hydra-init.service` ensures
proper permissions[2]. However, if `hydra-init.service` is not restarted on a
system-activation, the permissions of `/var/lib/hydra` will be set back to `0700`
by the activation script that runs on each activation.

This has lead to errors like this in `hydra-queue-runner` on my Hydra:

```
Sep 20 09:11:30 hydra hydra-queue-runner[306]: error (ignored): error: cannot unlink '/var/lib/hydra/build-logs/7h/dssz03gazrkqzfmlr5cprd0dvkg4db-squashfs.img.drv': Permission denied
Sep 20 09:11:30 hydra hydra-queue-runner[306]: error (ignored): error: cannot unlink '/var/lib/hydra/build-logs/b9/350vd8jpv1f86i312c9pkdcd2z56aw-squashfs.img.drv': Permission denied
Sep 20 09:11:30 hydra hydra-queue-runner[306]: error (ignored): error: cannot unlink '/var/lib/hydra/build-logs/kz/vlq4v9a1rylcp4fsqqav3lcjgskky4-squashfs.img.drv': Permission denied
Sep 20 09:11:30 hydra hydra-queue-runner[306]: error (ignored): error: cannot unlink '/var/lib/hydra/build-logs/xd/hkjnbbr9jp7364pkn8zpk9v8xapj2c-nix-2.4pre20210917_37cc50f.drv': Permission denied
Sep 20 09:11:30 hydra hydra-queue-runner[306]: error (ignored): error: cannot unlink '/var/lib/hydra/build-logs/zn/9df7225fl8p7iavqqfvlyay4rf0msw-nix-2.4pre20210917_37cc50f.drv': Permission denied
Sep 20 09:11:30 hydra hydra-queue-runner[306]: possibly transient failure building ‘/nix/store/7hdssz03gazrkqzfmlr5cprd0dvkg4db-squashfs.img.drv’ on ‘roflmayr’: error: creating directory '/var/lib/hydra/build-logs': Permission denied
Sep 20 09:11:30 hydra hydra-queue-runner[306]: will retry ‘/nix/store/7hdssz03gazrkqzfmlr5cprd0dvkg4db-squashfs.img.drv’ after 543s
```

Because of that, I decided to remove the `createHome = true;` setting and instead used
`systemd-tmpfiles`[3] which can not only ensure that certain directories
exist, but also proper permissions.

With this change, we can also get rid of the manual setup in
`hydra-init.service` since `systemd-tmpfiles` will be executed by
`switch-to-configuration` before *any* systemd service gets started. On
startup, `systemd-tmpfiles-setup.service` is invoked within
`sysinit.target` being reached, so when `hydra-init.service` gets called
in `multi-user.target`, the structure already exists.

[1] fa0d499dbf
[2] 3cec908738/hydra-module.nix (L260-L262)
[3] https://www.freedesktop.org/software/systemd/man/systemd-tmpfiles.html

hydra-module.nix

@@ -197,12 +197,15 @@ in
 
   config = mkIf cfg.enable {
 
+    systemd.tmpfiles.rules = [
+      "d ${baseDir} 0750 hydra hydra"
+    ];
+
     users.extraGroups.hydra = { };
 
     users.extraUsers.hydra =
       { description = "Hydra";
         group = "hydra";
-        createHome = true;
         home = baseDir;
         isSystemUser = true;
         useDefaultShell = true;
@@ -257,10 +260,6 @@ in
         };
         path = [ pkgs.utillinux ];
         preStart = ''
-          mkdir -p ${baseDir}
-          chown hydra.hydra ${baseDir}
-          chmod 0750 ${baseDir}
-
           ln -sf ${hydraConf} ${baseDir}/hydra.conf
 
           mkdir -m 0700 -p ${baseDir}/www