Improve get_last_build_version in tools/signing/nightly/sign-nightly

In get_last_build_version in tools/signing/nightly/sign-nightly we currently check for the current and previous day to find the latest build to sign. However, in some cases the build takes time and the latest build is from 2 or 3 days ago, and in this case it doesn't get signed.

We should change get_last_build_version to check for the last 5 days.