chore(pipeline): use infra.getBuildAgentLabel in agentSelector

[lemeurherve]

This change replaces the multiple if cases and the spotSelector function by a call to the infra.getAgentLabel global pipeline library function, as a centralised point to retrieve the appropriate agent template labels depending on the requested platform (no JDK implicated in those docker image builds), and on the controller where the job is running (ex: "fallback to from docker-highmem to linux" or "no spot/nonspot labels" on trusted.ci.jenkins.io).

Recommanded view: https://github.com/jenkinsci/docker-agents/pull/1137/changes?diff=split

Extracted from:

• chore(windows): build from Windows 2025 agents by default #1110
• PR resumed to this change when including this one

Refs:

• feat(infra.getBuildAgentLabel): add docker jobs labels jenkins-infra/pipeline-library#980
• feat(infra.getBuildAgentLabel): use a map for explicit parameters jenkins-infra/pipeline-library#985

Testing done

CI + infra.getBuildAgentLabel unit tests

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[timja]

I think doing it with the last number is a bit magic and could be clearer with more specific regexes