Only run Wasm tests on merge queue or workflow dispatch

[dhess]

The test suite is really slow when run via Wasm, so I think we should probably only run the test suite via Wasm when a PR hits the merge queue. We can also add a workflow_dispatch trigger so that the test suite can be run manually on any given PR. workflow_dispatch is a GitHub thing, and we're using Buildkite. Instead, we can use a special label on a PR when we want to run the Wasm tests prior to hitting the merge queue.

(There will probably be a very short time window during which you can label the PR before Buildkite starts its evaluation, so it will probably be necessary to rebuild the Wasm build after adding the label in most cases. I don't think this will be a big deal, though.)

[dhess]

This is partially implemented in #1226, but the label trick doesn't seem to work. I'll debug this later at a lower priority.

[dhess]

It might also be a good idea to add a label that skips the Wasm builds entirely. This would be useful for PRs that, say, make a doc change that doesn't affect the Wasm build at all. Our Nix builds will detect that no changes have been made to the Nix derivations and skip those builds, but our Wasm builds don't cache artifacts between runs.