ci(integration): Coordinate tested assets versions

[gadenbuie]

Three big changes:

1. We now run the R package tests using the shinylive assets we built and installed in the previous steps (before we were building shinylive but testing with the default assets version).
2. We now test r-shinylive in Quarto against the same version of shinylive as is used by py-shinylive. To do this, we needed to move the quarto integration test to happen immediately after installing the r-shinylive package, to ensure that we get the released shinylive assets. Later, we'll build and install the latest shinylive, which may have the same version number as the last release (but will be different).
3. Finally, we also test r/py-shinylive together in Quarto using the latest, custom built shinylive.

[schloerke]

We could use the py-shinylive assets version here, but that would lead to confusing testing. Instead, a proper and reasonable solution would be to update py-shinylive to support --assets-version in CLI to use different versions when exporting.