MGMT-15810: fix image missing nsenter executable

It seems like we were missing an explicit installation for the nsenter
executable which does not exist by default in stream9 base image.

The resulting errors are:
```
level=info msg="failed executing nsenter [--target 1 --cgroup --mount
--ipc --pid -- vgs --noheadings -o vg_name,pv_name], env vars
[PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HTTPS_PROXY= http_proxy= NO_PROXY= HTTP_PROXY= TERM=xterm
container=podman BUILD_LOGLEVEL=0
OPENSHIFT_BUILD_NAME=assisted-installer
OPENSHIFT_BUILD_NAMESPACE=ci-op-8g1imlzt https_proxy= no_proxy=
HOME=/root HOSTNAME=test-infra-cluster-3119ce36-master-2], error exec:
\"nsenter\": executable file not found in $PATH, waitStatus 0, Output
\"\""
```

(see
https://prow.ci.openshift.org/view/gs/origin-ci-test/pr-logs/pull/openshift_assisted-test-infra/2274/pull-ci-openshift-assisted-test-infra-master-e2e-metal-assisted/1706886740561629184)

This also adds this requirement to the controller image, although it
doesn't require it at the moment (but the dependency on ``ops.go`` means
that in the future we might still reuse more functions from that file).

Dockerfile.assisted-installer

@@ -14,6 +14,9 @@ RUN TARGETPLATFORM=$TARGETPLATFORM make installer
 
 FROM quay.io/centos/centos:stream9
 
+# required for nsenter
+RUN dnf install -y util-linux-core && dnf clean all
+
 COPY --from=builder /go/src/github.com/openshift/assisted-installer/build/installer /usr/bin/installer
 COPY --from=builder /go/src/github.com/openshift/assisted-installer/deploy/assisted-installer-controller /assisted-installer-controller/deploy