envoy: Downgrade go to 1.20

The build of envoy-deps currently fails with go 1.21 since the go.env file
is missing inside of the Bazel go SDK repository.

pkgs/top-level/all-packages.nix

···
   engelsystem = callPackage ../servers/web-apps/engelsystem { php = php81; };
 
   envoy = callPackage ../servers/http/envoy {
+    go = go_1_20;
     jdk = openjdk11_headless;
     gn = gn1924;
   };