feat: update nixpkgs and fix plpgsql-check build with gcc 15

With this new nixpkgs update come gcc 15, which introduced stricter C standard compliance. plpgsql-check fails to build due to this.
To fix the build, we explicitly set the C standard to gnu17.

Author: jfroche

flake.lock

@@ -40,6 +40,9 @@ let
         inherit hash;
       };
 
+      # Fix build with gcc 15
+      env.NIX_CFLAGS_COMPILE = "-std=gnu17";
+
       buildInputs = [ postgresql ];
 
       installPhase = ''