Dockerfile.rocm.ubi: add --mount=type=cache to final pip install

[dtrifiro]

Not having --mount=type=cache,target=/root/.cache/vllm results in the uv cache being included in the final image.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dtrifiro, NickLucche

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[openshift-ci]

@dtrifiro: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/smoke-test	b5312fc	link	true	/test smoke-test

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