Work around debvm/mmdebstrap incompatibility

Colin Watsonrequested to merge
cjwatson/debusine:debvm-mmdebstrap-incompatibility into devel

debvm 0.4.5 is incompatible with the version of mmdebstrap in bookworm (https://bugs.debian.org/1108354). Work around this by explicitly requiring the version of mmdebstrap in bookworm-backports.

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