Make pristine-tar mandatory for non-sloppy builds

This fails more explicitly than a later reject by the archive and
make things consistent with byzantium.

blessed_build_auto.groovy

@@ -86,8 +86,10 @@ pkgs.each { pkg ->
         } else { // regular blessed build
             build_args = "${default_build_args}"
             origtgz_cmd = """
-            git fetch origin pristine-tar || true
-            gbp export-orig --pristine-tar || gbp export-orig --no-pristine-tar
+            if ! grep -qs '(native)' debian/source/format; then
+              git fetch origin pristine-tar
+              gbp export-orig --pristine-tar
+            fi
             """
         }