Include version suffix in agent install command

What does this MR do and why?

Includes the agent version suffix (eg. -rc1) in the install command shown when registering an agent. Without the suffix, the frontend can show a non-existent image version and the install command will fail.

Screenshots or screen recordings

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How to set up and validate locally

1. Create an empty agent config file at .gitlab/agents/my-agent/config.yaml.
2. Go to Infrastructure -> Kubernetes clusters -> Connect a cluster, and select my-agent.
3. The install command should show image.tag=15.6.0-rc1, not 15.6.0.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

gitlab-org/charts/gitlab-agent#21 (closed)