Make tools/signing/nightly/sign-nightly sign mar files when there is no incrementals

tools/signing/nightly/sign-nightly currently check for the file sha256sums-unsigned-build.incrementals.txt to decide if a new version is ready to be signed. This means that when building incrementals failed, the mar files are not signed.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information