Set artifacts:name during script execution

It would be great if one could set the artifacts:name in the build script, i.e. by setting an environment variable and using that in the artifacts:name field, since the supplied predefined variables don't always cover exactly what I want.

I tried to do this on a windows cmd runner

script:
  - for /f %%i in ('git describe --tags') do set GIT_DESC=%%i
artifacts:
  name: Build_%GIT_DESC%
  paths:
    - somepath

and also tried putting it in after_script

script:
  - do stuff
artifacts:
  name: Build_%GIT_DESC%
  paths:
    - somepath
after_script:  
  - for /f %%i in ('git describe --tags') do set GIT_DESC=%%i

In both cases the variable is set correctly (as proved with set GIT_DESC), but the archive name is always Build_.zip.