Add workflow_dispatch to build.yml?

[annevk]

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch would allow us to manually run a workflow in the event we need it.

We kinda need it now and it seems impossible without creating commits.

[domenic]

I think you can just click the re-run button on GitHub jobs?

[domenic]

Interesting, you can only click that button for builds that have happened recently. (Last 30 days?)

I did that for all the specs for which it's possible, but yeah... let's add this.