fix: restore vfox and mise PR regression coverage

[yeshan333]

Summary

Make PR CI exercise the actual plugin changes across both vfox and mise, and fix the recent mise regression where Elixir source builds could not find erlc during install hooks.

Changes

• expand mise E2E tests PR triggers to cover plugin code and workflow changes
• link the checked-out repository as a local plugin in mise CI so PR runs validate the branch under review
• declare the Elixir plugin's Erlang hook dependency for mise and inject the resolved Erlang bin directory into the Elixir build command when needed
• expand vfox E2E tests PR triggers to cover plugin code and workflow changes
• make vfox PR runs fetch the plugin archive from the current PR head SHA instead of a branch ref
• fix the macOS runner condition in vfox E2E tests
• disable matrix fail-fast so platform regressions report independently

Testing

• latest PR checks now include all five regression jobs:
  • e2e_tests (ubuntu-22.04)
  • e2e_tests (macos-latest)
  • e2e_tests (windows-2022)
  • mise_e2e_tests (ubuntu-22.04)
  • mise_e2e_tests (macos-latest)
• previous mise PR run reproduced the macOS make: erlc: No such file or directory failure
• after the fix, the subsequent mise PR run passed on both Ubuntu and macOS

Context

The recent failures were not caused by a repo logic change alone. The breakage showed up after mise changed how vfox install-hook environments are built, which exposed this plugin's assumption that erlc would already be on PATH. This PR fixes that assumption and also makes sure future PRs run the full vfox and mise regression coverage against the code actually under review.

[yeshan333]

LGTM