boot/uki: Refactor .efi file collection #492

Job	Run time
build (bls, arch)	5m 29s
build (bls, rawhide)	6m 13s
build (bls, rhel9)	7m 58s
build (uki, arch)	5m 16s
build (bls, arch, xfs, none)	5m 7s
build (unified, fedora)	8m 21s
build (bls, fedora)	7m 57s
build (unified-secureboot, fedora)	26m 22s
build (bls, arch, ext4, none)	5m 5s
build (bls, fedora-compat)	6m 20s
build (uki, fedora)	8m 44s
	1h 32m 52s

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