Revert "ci: skip Debian unstable install test" #218

Job	Run time
yamllint	15s
system-testing (debian:unstable-slim)	3m 39s
unittest (debian:bullseye-slim)	31s
system-testing (ubuntu:jammy)	3m 52s
system-testing (ubuntu:mantic)	3m 38s
unittest (debian:bookworm-slim)	31s
install (debian:bullseye-slim)	14s
unittest (debian:testing-slim)	34s
install (debian:bookworm-slim)	16s
unittest (debian:unstable-slim)	35s
install (debian:testing-slim)	16s
unittest (ubuntu:jammy)	33s
unittest (ubuntu:mantic)	30s
install (debian:unstable-slim)	13s
install (ubuntu:jammy)	19s
install (ubuntu:mantic)	14s
	16m 10s

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