tools/setup-build: Properly emit release GN args

is_debug defaults to true, not false. Be explicit to fix the use of:

  ./tools/setup-build gn release

Change-Id: I225f0009f99901e6afeb0c504c98b2be961b7f76
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/91423
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

tools/setup-build

@@ -46,7 +46,7 @@ case $BUILD_SYSTEM in
         generate "gn gen out/active --args=is_debug=true"
       ;;
       "release")
-        generate "gn gen out/active"
+        generate "gn gen out/active --args=is_debug=false"
       ;;
       *)
         echo "invalid build type '${BUILD_TYPE}'"