GPU HIP CMake: More fixes for ROCm detection, wasn't working on my laptop

[davidrohr]

In the end, something is wrong if the CMAKE_PREFIX_PATH does not contain the ROCm folder in the env variable.
Not fully sure where it breaks, if inside CMake or inside the ROCm CMake files.
However, setting CMAKE_PREFIX_PATH as default inside the O2 CMake just leads to partially broken behavior where some parts are not found. No idea how to fix it.
Thus we change strategy:

• Check if CMAKE_PREFIX_PATH env variable is set and contains a ROCm path.
• If not, disable ROCm. If ROCm is required, fail the build.
• Derive all other ROCm paths from that PREFIX_PATH.

[github-actions]

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