Joshua's recipe fetches the client on the fly.
https://cs.chromium.org/chromium/infra/recipes-py/recipe_modules/isolated/api.py?l=24

That said for tester recipe, we want to pin the client from the builder recipe, for reproducibility. This is quite important!

We are using api.swarming_client.checkout(). Maybe this needs to be taught to do the right thing?

Here:
https://cs.chromium.org/chromium/build/scripts/slave/recipes/v8.py?q=v8.py&sq=package:chromium&dr&l=102

And here:
https://cs.chromium.org/chromium/build/scripts/slave/recipes/v8/flako.py?l=494

Joshua, I'd like to use swarming client from v8 builder.
At least, I want to ensure swarming client using cipd.

Do you have time to work on this CL?
https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1350350
If you are busy, can I take over the CL from you?

re #1, I want to have swarming here, not isolated.

re #2, I think we want to ensure go swarming client around there.

This was done by
https://chromium-review.googlesource.com/c/chromium/tools/build/+/1402224

But it seems we can't deprecate using the luci-py client in DEPS yet. On builder_tester type bots we still use swarming.py to trigger:
https://logs.chromium.org/logs/v8/buildbucket/cr-buildbucket.appspot.com/8924294573324730992/+/steps/trigger_tests/0/steps/s__trigger__Test262_-_no_variants/0/stdout

Let me file a separate bug.
https://bugs.chromium.org/p/chromium/issues/detail?id=921928

This bug is mainly to make luci-go client usable in v8's tester bots.