e2e: Fix the podman login to OCP internal registry

In OCP 4.16, dockercfg secret moved from the form of
"serviceaccount:xyz" to "<token>:xyz".

Use bash left trim operator (`${<variable>#<prefix>}`) to make it
explicit.

Signed-off-by: Andrea Panattoni <apanatto@redhat.com>

hack/run-e2e-conformance-virtual-ocp.sh

@@ -237,7 +237,11 @@ echo ${auth} > registry-login.conf
 internal_registry="image-registry.openshift-image-registry.svc:5000"
 pass=$( jq .\"image-registry.openshift-image-registry.svc:5000\".auth registry-login.conf  )
 pass=`echo ${pass:1:-1} | base64 -d`
-podman login -u serviceaccount -p ${pass:15} $registry --tls-verify=false
+
+# dockercfg password is in the form `<token>:password`. We need to trim the `<token>:` prefix
+pass=${pass#"<token>:"}
+
+podman login -u serviceaccount -p ${pass} $registry --tls-verify=false
 
 MAX_RETRIES=20
 DELAY_SECONDS=10