[!306] [cmake][bugfix] Fix handling of absolute paths for UFL files

Merge branch 'bugfix/absolute-path-to-uflfile' into 'master'

ref:extensions/dune-codegen This fixes [#143]

See merge request [extensions/dune-codegen!306]

  [#143]: gitlab.dune-project.org/NoneNone/issues/143
  [extensions/dune-codegen!306]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/306


Closes #143