test(e2e): use ubuntu:24.04 for the imex workload image (#437)

Switch the imex e2e asset and demo workload from debian:bookworm-slim to
ubuntu:24.04. Both containers only exercise /bin/sh (test -d, ls, check-fabric),
so behavior is unchanged; keeps the two manifests in sync.

Signed-off-by: Giulio Calzolari <gcalzolari@nvidia.com>

Author: giuliocalzo

docs/demo/nri-imex/imex-workload.yaml

@@ -26,7 +26,7 @@ spec:
         nvidia.com/gpu.present: "true"
       containers:
         - name: agent
-          image: debian:bookworm-slim
+          image: ubuntu:24.04
           command:
             - /bin/sh
             - -c

tests/e2e/go/assets/imex-workload.yaml

@@ -25,7 +25,7 @@ spec:
         nvidia.com/gpu.present: "true"
       containers:
         - name: agent
-          image: debian:bookworm-slim
+          image: ubuntu:24.04
           command:
             - /bin/sh
             - -c