Clean up Setup-Developer-Machine.ps1 and set $env:LcmRootDir

[papeh]

• Remove the do-nothing environment variable setup region
• Remove the junction to LCMRepo--it is located by an environment var
• Check for LibLCM existence in more places before cloning
• Set $env:LcmRootDir after finding or cloning LibLCM

CI-ready checklist

• Commit messages follow .github/commit-guidelines.md (subject ≤ 72 chars, no trailing punctuation; if body present, blank line then ≤ 80-char lines).
• Builds/tests pass locally (or I've run the CI-style build via build.ps1/test.ps1 or MSBuild).
• If this is core-developer AI-assisted work, I followed Docs/workflows/ai-pr-workflow.md and ran pr-preflight or the equivalent branch-readiness review before requesting review.
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate.
• All comments from an AI code reviewer have been considered (such as Devin)

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This change is 

[github-actions]

NUnit Tests

    1 files      1 suites   10m 58s ⏱️
5 778 tests 5 697 ✅ 81 💤 0 ❌
5 787 runs  5 706 ✅ 81 💤 0 ❌

Results for commit aa76301.

♻️ This comment has been updated with latest results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.06%. Comparing base (0ab90ff) to head (aa76301).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #1077       +/-   ##
=========================================
+ Coverage      0   38.06%   +38.06%     
=========================================
  Files         0     1499     +1499     
  Lines         0   350154   +350154     
  Branches      0    40241    +40241     
=========================================
+ Hits          0   133299   +133299     
- Misses        0   187573   +187573     
- Partials      0    29282    +29282     

see 1499 files with indirect coverage changes

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