Limit Jeremiah to 1 job

Even with only 2 jobs in parallel, Jeremiah can't keep up with only 16GB
of RAM. Gonna have to push to get some more memory for it. This is
painful!

hosts/jeremiah/default.nix

@@ -76,11 +76,11 @@
   services = {
     gitlab-runner = {
       enable = true;
-      settings.concurrent = 3;
+      settings.concurrent = 5;
       services = {
         shell = {
           executor = "shell";
-          limit = 5;
+          limit = 1;
           authenticationTokenConfigFile = config.age.secrets.runner-reg.path;
           environmentVariables = {
             EFI_DIR = "${pkgs.OVMF.fd}/FV/";