Unable to point to a static, offline copy of dependencies

[bwarden]

In order to package this software for a distribution, or any software that requires it as part of its own build (such as DPC++ Compiler), it is essential that all necessary source code can be downloaded and provided offline, such that the build will not attempt to retrieve anything from the internet.

https://github.com/oneapi-src/unified-runtime/blob/6b5dec67adb2d95e0f92fd8f69e5d3a73bf67654/cmake/FetchLevelZero.cmake#L112-L124

Ideally, cmake's own methods such as find_package should be used, but lacking that, I should be able to provide variables that point to wherever I have unpacked compute-runtime.

[alycm]

Note this is the same use-case, but with different suggestions, as #2494.