Install lxc-templates in autopkgtest images, again

It was demoted from Recommends to Suggests in lxc, yesterday: lxc-team/lxc@c500e93d

This should fix autopkgtest jobs.

Example failure:

autopkgtest [10:01:26]: starting date and time: 2024-05-03 10:01:26+0000
78autopkgtest [10:01:26]: version 5.34
79autopkgtest [10:01:26]: host runner-f0fdd533-project-87790-concurrent-0; command line: /usr/bin/autopkgtest --no-built-binaries '--setup-commands=echo '"'"'/builds/cjwatson/debusine/debian/output/debusine_0.2.2+salsaci+20240503+544_amd64.changes bookworm/amd64'"'"' > /var/tmp/debci.pkg 2>&1 || true' '--setup-commands=echo '"'"'Acquire::Retries "10";'"'"' > /etc/apt/apt.conf.d/75retry 2>&1 || true' --test-name=integration-tests-task-simplesystemimagebuild --user debci --apt-upgrade --output-dir=/builds/cjwatson/debusine/debian/output/debci /builds/cjwatson/debusine/debian/output/debusine_0.2.2+salsaci+20240503+544_amd64.changes -- lxc --sudo --name ci-124-724f7239 autopkgtest-bookworm-amd64
80lxc-copy: autopkgtest-bookworm-amd64: ../src/lxc/parse.c: lxc_file_for_each_line_mmap: 78 No such file or directory - Failed to open file "/usr/share/lxc/config/debian.common.conf"
81lxc-copy: autopkgtest-bookworm-amd64: ../src/lxc/parse.c: lxc_file_for_each_line_mmap: 129 Failed to parse config file "/builds/cjwatson/debusine/lxc/autopkgtest-bookworm-amd64/config" at line "lxc.include = /usr/share/lxc/config/debian.common.conf"
82Failed to load config for autopkgtest-bookworm-amd64
83autopkgtest-virt-lxc: WARNING: retrying lxc-copy because it failed with exit status 1
84lxc-copy: autopkgtest-bookworm-amd64: ../src/lxc/parse.c: lxc_file_for_each_line_mmap: 78 No such file or directory - Failed to open file "/usr/share/lxc/config/debian.common.conf"
85lxc-copy: autopkgtest-bookworm-amd64: ../src/lxc/parse.c: lxc_file_for_each_line_mmap: 129 Failed to parse config file "/builds/cjwatson/debusine/lxc/autopkgtest-bookworm-amd64/config" at line "lxc.include = /usr/share/lxc/config/debian.common.conf"
86Failed to load config for autopkgtest-bookworm-amd64
87<VirtSubproc>: failure: lxc-copy with exit status 1
88autopkgtest [10:01:32]: ERROR: testbed failure: unexpected eof from the testbed
89