Switch to external composefs #2237

Job	Run time
build-integration	4m 9s
Code style	50s
Build (Fedora)	1m 27s
Build - FCOS minimal	1m 47s
Debian Stable with sign-ed25519 and FUSE 2	7m 44s
Debian Stable with curl, sign-ed25519, no gpgme, FUSE 3	7m 1s
Debian Stable 32 bit	7m 54s
Debian Stable without libsystemd	7m 15s
Debian Testing	7m 46s
Debian Testing with libsoup3	7m 8s
Debian stable + static-prepareroot	7m 1s
Ubuntu Latest Release	7m 1s
Privileged testing	42s
	1h 7m 45s

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