[bootlin/training-materials] be615b: github: add workaround for checkout issue

[Clément Léger]

  Branch: refs/heads/master_fix_action
  Home:   https://github.com/bootlin/training-materials
  Commit: be615b667573c8a5edcaa72a37c4bf3f87ecb21a
      https://github.com/bootlin/training-materials/commit/be615b667573c8a5edcaa72a37c4bf3f87ecb21a
  Author: Clément Léger <clement.leger at bootlin.com>
  Date:   2022-05-23 (Mon, 23 May 2022)

  Changed paths:
    M .github/actions/build-material-action/Dockerfile
    M .github/workflows/training-materials.yml

  Log Message:
  -----------
  github: add workaround for checkout issue

There seems to be a bug with v2 action when used with Ubuntu 20-04 [1].
Unfortunately, there is no solution yet and a worarkound should be
used.

[1] https://github.com/actions/checkout/issues/760

Signed-off-by: Clément Léger <clement.leger at bootlin.com>