steamPackages.runtime: use mirrors, add my mirror

pkgs/build-support/fetchurl/mirrors.nix

···
     ftp://ftp.halifax.rwth-aachen.de/pub/OpenBSD/
     ftp://mirror.switch.ch/pub/OpenBSD/
   ];
+
+  # Steam Runtime mirrors
+  steamrt = [
+    http://repo.steampowered.com/steamrt/
+    https://abbradar.net/steamrt/
+  ];
 }

pkgs/games/steam/update-runtime.py

···
 	out.write("    rec {\n")
 	out.write("      name = \"%s\";\n" % file_name)
 	out.write("      md5 = \"%s\";\n" % md5.strip())
+	out.write("      url = \"%s\";\n" % file_url.replace(REPO, "mirror://steamrt", 1))
 	out.write("      source = fetchurl {\n")
-	out.write("        url = \"%s\";\n" % file_url)
-	out.write("        inherit md5;\n")
+	out.write("        inherit url md5;\n")
 	out.write("        name = \"%s\";\n" % file_shortname)
 	out.write("      };\n")
 	out.write("    }\n")