ci: use autopkgtest from ubuntu-backports in semaphore jobs

noble has a new enough autopkgtest, so use that instead of cloning from
git, as there are often transient regressions, and salsa availability
is also spotty

(cherry picked from commit 0d033931a4)
(cherry picked from commit 681791eb56)

.semaphore/semaphore-runner.sh

@@ -12,7 +12,6 @@ ARCH="${ARCH:-amd64}"
 CONTAINER="${RELEASE}-${ARCH}"
 CACHE_DIR=/var/tmp
 TMPDIR=/var/tmp
-AUTOPKGTEST_DIR="${CACHE_DIR}/autopkgtest"
 # semaphore cannot expose these, but useful for interactive/local runs
 ARTIFACTS_DIR=/tmp/artifacts
 # shellcheck disable=SC2206
@@ -76,14 +75,12 @@ for phase in "${PHASES[@]}"; do
             # remove chrome repo, we don't need it
             sudo rm -rf /etc/apt/sources.list.d/google-chrome.sources
 
-            # enable backports for latest LXC
+            # enable backports for latest LXC and autopkgtest
             echo "deb http://archive.ubuntu.com/ubuntu $UBUNTU_RELEASE-backports main restricted universe multiverse" | sudo tee -a /etc/apt/sources.list.d/backports.list
             sudo apt-get -q update
-            sudo apt-get install -y -t "$UBUNTU_RELEASE-backports" lxc
+            sudo apt-get install -y -t "$UBUNTU_RELEASE-backports" lxc autopkgtest
             sudo apt-get install -y python3-debian git dpkg-dev fakeroot python3-jinja2
 
-            [ -d "$AUTOPKGTEST_DIR" ] || git clone --quiet --depth=1 https://salsa.debian.org/ci-team/autopkgtest.git "$AUTOPKGTEST_DIR"
-
             create_container
         ;;
         RUN)
@@ -119,7 +116,7 @@ EOF
             # now build the package and run the tests
             rm -rf "$ARTIFACTS_DIR"
             # autopkgtest exits with 2 for "some tests skipped", accept that
-            sudo TMPDIR=/var/tmp "$AUTOPKGTEST_DIR/runner/autopkgtest" --env DEB_BUILD_OPTIONS="noudeb nostrip nodoc optimize=-lto" \
+            sudo TMPDIR=/var/tmp autopkgtest --env DEB_BUILD_OPTIONS="noudeb nostrip nodoc optimize=-lto" \
                                                        --env DPKG_DEB_COMPRESSOR_TYPE="none" \
                                                        --env DEB_BUILD_PROFILES="pkg.systemd.upstream noudeb nodoc" \
                                                        --env TEST_UPSTREAM=1 \