CI: split job name parts to extra variables

Problem to solve

It would be nice when CI provides the parts of the jobname in dedicated environment variables. E.g. when having

build:linux:fedora:
  script:
  - set

the output could be like

CI_BUILD_NAME=build:linux:fedora
CI_BUILD_NAME_0=build
CI_BUILD_NAME_1=linux
CI_BUILD_NAME_2=fedora

These variables should be usable in places like image: or artifacts:name:.

With using yaml anchors, this will ease setup of test matrices with different setups.