ubuntu: more exact match on squashfs file, containerfile: use focal

This is a squash of two changes that have unfortunately simultaneously
broken the gate.

The functests are failing with

 sha256sum: bionic-server-cloudimg-amd64.squashfs.manifest: No such file or directory

I think what has happened here is that the SHA256 sums file being used
has got a new entry "bionic-server-cloudimg-amd64.squashfs.manifest"
which is showing up in a grep for
"bionic-server-cloudimg-amd64.squashfs".  sha256 then tries to also
check this hash, and has started failing.

To avoid this, add an EOL marker to the grep so it only matches the
exact filename.

Change I7fb585bc5ccc52803eea107e76dddf5e9fde8646 updated the
containerfile tests to Jammy and it seems that cgroups v2 prevents
podman running inside docker [1].  While we investigate, move this
testing back to focal.

[1] containers/podman#14884
Change-Id: I1af9f5599168aadc1e7fcdfae281935e6211a597

(cherry picked from commit 78d3895)

diskimage_builder/elements/ubuntu/root.d/10-cache-ubuntu-tarball

@@ -41,10 +41,10 @@ function get_ubuntu_tarball() {
             # downloading the larger image), and then if the sums still fail retry
             # the image.
             $TMP_HOOKS_PATH/bin/cache-url -f $SHA256SUMS $CACHED_SUMS
-            if ! grep "$BASE_IMAGE_FILE" $CACHED_SUMS | sha256sum --check - ; then
+            if ! grep "${BASE_IMAGE_FILE}$" $CACHED_SUMS | sha256sum --check - ; then
                 $TMP_HOOKS_PATH/bin/cache-url -f \
                     $DIB_CLOUD_IMAGES/$BASE_IMAGE_FILE $CACHED_FILE
-                grep "$BASE_IMAGE_FILE" $CACHED_SUMS | sha256sum --check -
+                grep "${BASE_IMAGE_FILE}$" $CACHED_SUMS | sha256sum --check -
             fi
         fi
         popd