This will lead you to this GitHub proposal, which will do exactly what we're set to do here, i.e., build and test:
GitHub will preview the .yml file it will create. For this scope, you don't have to change anything (except maybe the name from "Rust" to something else — I used "test").
Commit the change clicking on the button that will appear on the right and the file will be created:
Now, if you go to Actions again, you will see GitHub creating your .yml file. For me, it took around 2 minutes.
And that's it. From now on, every time you push against the repository, GitHub will run the tests for you.
To see the result of the commit above, see the Build log here.