hello-go: fix longDescription (GOOS and GOARCH specification)

This reverts commit e53afdda6e01c4886d58e86c2f84bcccacf4a744.

pkgs/by-name/he/hello-go/package.nix

···
       Specify target platform by setting GOOS and GOARCH:
 
       ```nix
-      hello-go.overrideAttrs {
-        GOOS = "linux";
-        GOARCH = "arm64";
-      }
+      hello-go.overrideAttrs (previousAttrs: {
+        env = previousAttrs.env or { } // {
+          GOOS = "linux";
+          GOARCH = "arm64";
+        };
+      })
       ```
 
       See https://pkg.go.dev/internal/platform#pkg-variables for a list