Ubuntu 64-bit "Failed to fetch file [..] binary-i386/Packages" error while updating apt repos

I had the same issue using my own apt-mirror server to update the git-core package.

In my case, I resolved the issue by adding [arch=amd64] in the original entry in the /etc/apt/sources.list.

Example:
Original: deb http://192.168.111.222/mirror/git-core precise main
Solution: deb [arch=amd64] http://192.168.111.222/mirror/git-core precise main

The details can be found at: https://wiki.ubuntu.com/MultiarchSpec#apt_sources

In your case, you need to edit /etc/apt/sources.list.d/apt-build.list.

As suggested by the accepted answer, I had to edit the contents of:

/etc/apt/sources.list.d/apt-build.list

From:

deb file:/var/cache/apt-build/repository apt-build main

to:

deb [arch=amd64] file:/var/cache/apt-build/repository apt-build main

Note that the error came solely from apt-build.list. No changes to /etc/apt/sources.list were needed to fix the problem.