tests/kernel-replace: use dnf repoquery to get kernel version

When testing from the c9s repos, don't hardcode a kernel version since
it'll eventually cycle out of the mirrors. Instead, just query the repo
to get the latest kernel.

Since we need the info we queried across reboots and we don't want to
query it each time (and possibly get different answers), persist it in
`/var` instead.

Ideally we'd do this for FCOS too, but we need to wait until the f41
rebase, which will bring dnf. (Or... could do it in a toolbox, but that
seems overkill.) But anyway for now, the kernel from the stable Fedora
repo shouldn't ever be GC'ed.

tests/kola/rpm-ostree/kernel-replace

@@ -48,20 +48,37 @@ image=oci:${image_dir}
 image_pull=ostree-unverified-image:$image
 tmp_imagedir=${image_dir}-tmp
 arch=$(arch)
-case "$OS_ID" in
-  rhcos|scos|rhel|centos)
-    kver="5.14.0-472.el9.${arch}"
-    url="https://mirror.stream.centos.org/9-stream/BaseOS/${arch}/os/Packages/kernel"
-    ;;
-  fedora)
-    kver="6.9.8-200.fc40.${arch}"
-    url="https://kojipkgs.fedoraproject.org//packages/kernel/6.9.8/200.fc40/${arch}/kernel"
-    ;;
-  *)
-    echo "Unknown OS_ID: ${OS_ID}"
-    exit 1
-    ;;
-esac
+
+# we only want to query repos once but need the info on multiple boots, so write out state to /var
+if [ ! -f /var/kola-kernel.evr ]; then
+  case "$OS_ID" in
+    rhcos|scos|rhel|centos)
+      c9s_mirror="https://mirror.stream.centos.org/9-stream/BaseOS/${arch}/os"
+      # we're probably already on CentOS and running the latest kernel. so
+      # here we need to instead pick whatever the latest that's *not* the same
+      kver=$(dnf repoquery kernel --qf '%{EVR}' --repofrompath=tmp,"${c9s_mirror}" --disablerepo '*' --enablerepo tmp | \
+                grep -v "$(rpm -q kernel --qf '%{EVR}')" | tail -n1)
+      kver="${kver}.${arch}"
+      url="${c9s_mirror}/Packages/kernel"
+      ;;
+    fedora)
+      # to get the version to use: koji latest-pkg f40 kernel
+      # XXX: once we can rely on dnf in FCOS, use repoquery also here
+      kver="6.8.5-301.fc40.${arch}"
+      url="https://kojipkgs.fedoraproject.org//packages/kernel/6.8.5/301.fc40/${arch}/kernel"
+      ;;
+    *)
+      echo "Unknown OS_ID: ${OS_ID}"
+      exit 1
+      ;;
+  esac
+  echo "$kver" > /var/kola-kernel.evr
+  echo "$url" > /var/kola-kernel.url
+else
+  kver=$(cat /var/kola-kernel.evr)
+  url=$(cat /var/kola-kernel.url)
+fi
+
 case "${AUTOPKGTEST_REBOOT_MARK:-}" in
   "")
     # Take the existing ostree commit, and export it to a container image, then rebase to it.