Add base-browser nightly mar signing key
We need to add the nightly mar signing key as toolkit/mozapps/update/updater/nightly_aurora_level3_primary.der and toolkit/mozapps/update/updater/nightly_aurora_level3_secondary.der.
We need to add the nightly mar signing key as toolkit/mozapps/update/updater/nightly_aurora_level3_primary.der and toolkit/mozapps/update/updater/nightly_aurora_level3_secondary.der.