installation/testing of CIL on docker and VM

[KrisThielemans]

At present, we install CIL as follows:

• docker: via conda in the docker image via SIRF-Exercises/environment.yml and have BUILD_CIL=OFF
• VM: using BUILD_CIL=ON
• GHA: using BUILD_CIL=ON

Aside from being non-uniform and confusing, it also means that CIL tests are not run on Docker (as we use ctest).

[KrisThielemans]

so: let's switch BUILD_CIL=ON and disable reading the sirf-exercises environment.yml and use requirements.yml in the docker folder. Of course, this means checking that whatever is in there is in the former (except CIL!).

[KrisThielemans]

Note that CIL is now tested on docker via #838. The rest of the confusing situation still applies.