Fix the image tag (#3446)

Fixes #3445

pbench_demo uses an image downloaded from Quay.io. It assumes that the
image will contain `pbench-generate-token` which is available
currently but it is deprecated and will not be available in the
future.  In particular, the specified image
`quay.io/pbench/pbench-agent-all-fedora-36:all` does *not* contain the
script. We need to make use
`quay.io/pbench/pbench-agent-all-fedora-36:b0.72` instead.

PBENCH-1129

contrib/containerized-pbench/pbench_demo

@@ -16,7 +16,7 @@ alias pbench="$(git rev-parse --show-toplevel)"/contrib/containerized-pbench/pbe
 
 FIOTEST=${PWD}/fiotest
 export PB_AGENT_PODMAN_OPTIONS="--pull newer -v ${FIOTEST}:/fiotest:z"
-export PB_AGENT_IMAGE_NAME=quay.io/pbench/pbench-agent-all-fedora-36:main
+export PB_AGENT_IMAGE_NAME=quay.io/pbench/pbench-agent-all-fedora-36:b0.72
 
 mkdir -p "${FIOTEST}"