[BUG] Missing license_file in CRAN recipes

[roaldarbol]

Describe the bug
CRAN packages don't ship with a license file, but license_file is a required field. So right now recipes for R packages created with grayskull are failing (see e.g. conda-forge/staged-recipes#24987).

To Reproduce
Steps to reproduce the behavior:

1. Generate recipe with grayskull cran sleepr
2. Attempt to merge to staged-recipes (or see the above PR)

Expected behavior
To create a valid license_file field.

Outputs
If applicable, add the output to help explain your problem.

Environment:

• grayskull --version: 2.5.3

Additional context
This has previously been solved on conda-build - see conda-forge/staged-recipes#3234, so all the relevant information should be there I would imagine. See discussion about it here: conda-forge/staged-recipes#3234.