[SYCL][E2E] Add an env var to run on Windows

[bso-intel]

SYCL_ENABLE_DEFAULT_CONTEXTS=1 is needed to run on Windows.

[bso-intel]

@KseniyaTikhomirova
ping

[bso-intel]

@intel/llvm-gatekeepers
ping

[AlexeySachkov]

I'm confused by this PR. #12599 from @cperkinsintel says that default context is now enabled by default on Windows and I don't see any mentions of its revert.

Is this env variable needed because we haven't updated user-facing APIs related to default context, so they still don't work by default on Windows?

llvm/sycl/test-e2e/AbiNeutral/device-info.cpp

Lines 69 to 70 in 46571e3

	gDevPool.contexts[0] = std::make_unique<sycl::context>(
	gDevPool.devices[0]->get_platform().ext_oneapi_get_default_context());