ci: remove results directory content only with piglit runners

ci: remove results directory content only with piglit runners

Removing the directory itself can be problematic with certain runner
strategies (B2C).

v2:
  - Better deleting pattern matching since the previously used one was
    problematic and not pointed out by /bin/sh, as noticed by Emma.

v3:
  - Check that the results directory exists before attempting to
    delete its content.

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Martin Peres <martin.peres@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10856>