ci: install pkg to user home on runners without passwordless sudo

[Fedr]

Summary

Lets test-distribution / macos-test run on runners without passwordless sudo (e.g. our self-hosted macOS fleet) by installing the .pkg into ~/Library via installer -target CurrentUserHomeDirectory instead of sudo installer -target /.

Two coordinated changes:

• macos/Distribution.xml — adds <domains enable_currentUserHome="true" enable_localSystem="true"/>. macOS rejects -target CurrentUserHomeDirectory by default unless the pkg explicitly opts into the currentUserHome domain. enable_localSystem="true" keeps the existing default system-wide install path, so end users are unaffected and the GUI installer still defaults to /Library.

• .github/workflows/test-distribution.yml — macos-test Install Pkg step branches on runner.environment:

  • github-hosted → keeps sudo installer -pkg ... -target / into /Library/Frameworks/... (production install path stays exercised end-to-end).
  • self-hosted → installer -pkg ... -target CurrentUserHomeDirectory into ~/Library/Frameworks/... (no sudo).
  • The downstream steps (Run MeshViewer, Show meshconv help, brew install -< macos.txt, C/C++ example builds) are made install-location-agnostic by capturing MESHLIB_FW once and putting $MESHLIB_FW/bin on $GITHUB_PATH. CMake's find_package(MeshLib CONFIG REQUIRED) in the examples auto-discovers the framework via CMAKE_SYSTEM_FRAMEWORK_PATH (~/Library/Frameworks is in the macOS default), so no -DCMAKE_PREFIX_PATH= is needed.

Why this came up

PR #6138 expanded the macos-test matrix to include the self-hosted runners macos-arm-build-12 and macos-x64-build, and both failed at "Install Pkg" with sudo: a password is required — the runner service on those hosts runs as a non-admin user, so sudo installer never succeeds. This PR is one of the two follow-ups discussed in #6138's thread (the other is #6141, which pinned CMAKE_OSX_DEPLOYMENT_TARGET=12.7 and is now in master). With both in master, #6138 can be rebased and the full 7-runner matrix should go green.

CI scope

Only macOS distribution testing is affected. Non-mac jobs are disabled via labels. full-ci is set so upload_artifacts is true and test-distribution actually runs from build-test-distribute.yml.

Limitations of this PR's own CI run

Master's current macos-test matrix has only the two GitHub-hosted runners (macos-15-intel, macos-latest). Both take the github-hosted branch of the new if [ "${{ runner.environment }}" = "github-hosted" ], so the new self-hosted branch is not exercised by this PR's own CI. What this PR's run did verify:

• the new branching, env-var capture, and $GITHUB_PATH plumbing don't break the existing github-hosted path,
• the <domains> addition to Distribution.xml doesn't break .pkg generation or system-wide installs.

The self-hosted branch is verified once #6138 rebases on top and its matrix expansion lands.

Test plan

Verified in run 26240775524:

• test-distribution / macos-test (x64, macos-15-intel, *x64.pkg) still passes — .pkg installs to /Library via sudo (the github-hosted code path).
• test-distribution / macos-test (arm64, macos-latest, *arm.pkg) still passes — same code path.
• No downstream step regresses on the env-var captured paths (Run MeshViewer, Show meshconv help, examples builds).
• upload-distributions succeeds — the <domains> addition to Distribution.xml doesn't break productbuild.
• All 3 macos-build-test variants compile cleanly (x64-Release, arm64-Debug, arm64-Release).
• Follow-up after merge: rebase ci: widen test-distribution macos-test, rebuild cpr on arm64 only #6138, confirm the self-hosted rows (macos-arm-build-12, macos-x64-build) reach "Install Pkg" without sudo and proceed through MeshViewer / meshconv / examples.