chore: improve env var scopes and retrieve `VERSION` in build.ps1

[lemeurherve]

This PR improves environment variable scopes, allows retrieving VERSION from env var in build.ps1 so it can benefits the new scoping, and shortens agentSelector comments in the Jenkinsfile.

By doing so, it removes any interpolation in the last sh and powershell code blocks.

Fixes:

• 423

Extracted from:

• 415

Testing done

CI

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[lemeurherve]

So sorry @timja just noticed your new review after repushing to remove the dangling -VersionTag in the last buid.ps1 call 😓