qa-code-coverage: update fallback files fetching command

This patch replaces `wget` for `fetch_file()` function which
allows to use to local and remote files.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: Iede7736ef52a0be5dbe1a6151f59853bebf6e810
1 file changed