gams: move assert behind meta.license check

This now errors with the "allow unfree" error first - and only when
that's passed it asserts the specific `licenseFile` option.

This ensures that CI will not produce an eval warning for the assert
once we turn that on.

pkgs/tools/misc/gams/default.nix

@@ -8,8 +8,6 @@
   optgamsFile ? null,
 }:
 
-assert licenseFile != null;
-
 stdenv.mkDerivation rec {
   version = "25.0.2";
   pname = "gams";
@@ -23,6 +21,7 @@ stdenv.mkDerivation rec {
   dontBuild = true;
 
   installPhase =
+    assert licenseFile != null;
     ''
       mkdir -p "$out/bin" "$out/share/gams"
       cp -a * "$out/share/gams"