tf-m-nightly-code-coverage: Set to run weekly on Sat

After all failure causes for this job were addressed. Changed to run
weekly instead of previous daily to conserve resources.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ie938f128bf073da755c3b836b8326542d9c058e5
1 file changed