maintainers: add gvolpe

maintainers/maintainer-list.nix

···
     githubId = 6768842;
     name = "Joris Guyonvarch";
   };
+  gvolpe = {
+    email = "[e-mail]";
+    github = "gvolpe";
+    githubId = 443978;
+    name = "Gabriel Volpe";
+  };
   hakuch = {
     email = "[e-mail]";
     github = "hakuch";