smoketest: 50 removes packages the archive doesn't have before going back

clang-libs and llvm-lit were split off clang and llvm after the archive
date, so the old clang and llvm conflicted with their files and the
downgrade failed, skipping the scenario. Remove packages from Arch's repos
that the archive doesn't have yet before downgrading.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

smoketest/scenarios/50-stale-system.sh

@@ -9,11 +9,22 @@ scenario() {
   date="$(date -d '30 days ago' +%Y/%m/%d)"
 
   # Go back to Arch as it was a month ago, from the Arch Linux Archive, which
-  # is also a mirror that stopped syncing a month ago
+  # is also a mirror that stopped syncing a month ago. Packages from Arch's
+  # repos that the archive doesn't have yet, often split off since (e.g.
+  # clang-libs from clang), go first: their files conflict with the old
+  # packages'. update installs them again
   run_in_vm downgrade "set -e
+    pacman -Qqn | sort > ~/native-now
     echo 'Server = https://archive.archlinux.org/repos/$date/\$repo/os/\$arch' \
       | sudo tee /etc/pacman.d/mirrorlist
-    sudo pacman -Syyuu --noconfirm"
+    sudo pacman -Syy
+    newer=\"\$(pacman -Qqm | sort | comm -12 ~/native-now -)\"
+    rm ~/native-now
+    if [ -n \"\$newer\" ]; then
+      echo \"Not in the archive yet, removing: \"\$newer
+      sudo pacman -Rdd --noconfirm \$newer
+    fi
+    sudo pacman -Suu --noconfirm"
   [ "$STATUS" -eq 0 ] || skip "couldn't downgrade to Arch as of $date (see $LAST_LOG)"
   vm_reboot
   kernel_before="$(vm_value 'uname -r')"