CI: Enable nightly tests

[leofang]

Steps:

1. Add another workflow triggered every night
2. Reuse the latest artifacts generated from the main branch (as seen at the time of tests)
3. Only run tests and skip the build stage (as we already have the artifacts)

Need to cover more driver versions (#293), GPU species, and number of GPUs. If we have any slow tests (maybe benchmarks from #109?), they should be run at this stage.

[leofang]

To add:

• benchmarks (Update benchmarks and enable them #407)