add robwalt to maintainer list

maintainers/maintainer-list.nix

···
     githubId = 1069318;
     name = "Robin Lambertz";
   };
+  robwalt = {
+    email = "[e-mail]";
+    github = "robwalt";
+    githubId = 26892280;
+    name = "Robert Walter";
+  };
   roconnor = {
     email = "[e-mail]";
     github = "roconnor";