tests: build test and cache both versions

Author: samrose

docker/nix/build_nix.sh

@@ -6,5 +6,7 @@ if [ -d "/workspace" ]; then
     cd /workspace
 fi
 nix build .#psql_15/bin -o psql_15
+nix build .#psql_16/bin -o psql_16
 nix flake check -L 
 nix copy --to s3://nix-postgres-artifacts?secret-key=nix-secret-key ./psql_15
+nix copy --to s3://nix-postgres-artifacts?secret-key=nix-secret-key ./psql_16