Update pr-realm-js workflow to run Android tests on Ubuntu

[kraenhansen]

What, How & Why?

With inspiration from @nielsenko via https://github.com/realm/realm-dart/commit/2d93a4f3 this should speed-up running our Android tests.

As a baseline, the Android tests are currently taking:

• 23m 43s
• 23m 11s
• 23m 26s

[coveralls-official[bot]]

coverage: 85.346%. remained the same when pulling 3e406012ff58b183709cfe7e4aa4af17b901acd6 on kh/ci-test-android-on-linux into 795413316e5c8811232d8a82729c1fe51879fdb7 on main.

[kraenhansen]

For some strange reason, the runner doesn't exit when the tests are done 🤔 I would have to debug this further, perhaps by running the harness in a docker locally ...