Fix `ISLE_ENTROPY_FILENAME` option in CMakeLists.txt

[disinvite]

ISLE_ENTROPY_FILENAME selects the generated entropy filename to include for each thread of the entropy build in the CI. This is working in its current state, but it's not good CMake syntax. For one thing, it shouldn't be an option() because it's not a boolean.

[madebr]

You probably want

set(ISLE_ENTROPY_FILENAME "entropy.h" CACHE STRING "Entropy header filename")