Describe the bug
If your Cube directories are not in the default paths and you use relative paths for STM32_CUBE_<FAMILY>_PATH it will fail to find CMSIS.

Sources to reproduce
Please provide link to repository showing the problem.

• In main branch

Steps to reproduce

1. Download of fetch Cube code from examples/fetch-cube
2. Place it somewhere other than the default location of /opt/STM32Cube<FAMILY>
3. Go to root/examples/blinky
4. Create a build folder and cd build
5. Run cmake .. -DSTM32_CUBE_F4_PATH=<absolute-path>/stm32-cmake/thirdParty/STM32CubeF4
6. This builds successfully.
7. rm -rf ./* in build
8. Run cmake .. -DSTM32_CUBE_F4_PATH=<relative-path>/stm32-cmake/thirdParty/STM32CubeF4 leads to
   CMake Error at /usr/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:290 (message): Could NOT find CMSIS (missing: CMSIS_INCLUDE_DIRS CMSIS_SOURCES STM32F4)

Expected behavior
Expected behavior is that this project should be able to handle relative paths or at least mention it in big bold letters in the documentation.

Screenshots
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Environment (please complete the following information):

• OS: Linux x86_64
• Compiler: arm-none-eabi-gcc 15.2.Rel1
• stm32-cmake: ecc5acc
• cmake: 4.2.2

Additional context
Add any other context about the problem here.