nixos/lib/make-ext4-fs: fix after mkfs.ext4 refactoring
nixos/lib/make-ext4-fs: fix after mkfs.ext4 refactoring

In 9ac1ab10c963a86457c2c9b1edf31542ff3737cd this library function was
refactored to use mkfs.ext4 instead of cptofs. There are two problems:

If populateImageCommands would create no files (which is fine), a cp
invocation would fail due to missing source arguments.

Another problem is that mkfs.ext4 relies on fakeroot to have sane
uid/gids in the generated filesystem image. This currently doesn't
work for cross compiling.

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