[CI] Prebuild E2E tests as part of build workflow

[aelovikov-intel]

This will allow us to save time on runner setup and toolchain download/unpacking.

[uditagarwal97]

Does it mean that whenever we call .github/workflows/sycl-linux-build.yml, we also build E2E tests? IIRC, we don't use pre-built test binaries in post-commit and nightly.

[aelovikov-intel]

Yes, we do, and no, we don't. That said I imagine we can start doing so relatively soon. Alternatively, we can make this addition optional, but I thought that build-only mode of e2e tests isn't that different from check-sycl that we do unconditionally everywhere (well, if sycl is modified in pre-commit). Writing this, I can even imagine re-unification of sycl/test and sycl/test-e2e now!

[uditagarwal97]

I would personally be of the opinion that we make this addition optional - don't build E2E tests by default when sycl-linux-build.yml is called, unless the caller explicitly asks for it.
Once the "split-test" mode is mature enough that we use it for all workflows and other runners as well, we can build E2E tests by default, just like sycl/test.

[aelovikov-intel]

I think it will just complicate implementation with very little benefit... Nightly/post traffic is several times smaller than pre-commit (because PR usually take several iterations). Also, unlike pre-commit, nightly/post don't skip any check-* at build stage, so having e2e built there takes less percentage of the overall duration.