Pin MoonBit toolchain in CI

[mizchi]

Summary

• add a reusable local action for installing the repository's MoonBit toolchain
• pin CI to moonbit v0.10.6+80dc50f24
• use the same setup from CI, JS build, randomized compatibility, Pages, release, and Copilot workflows
• report the installed compiler version in every job

Root cause

The official latest channel currently installs moon 0.1.20260807 / moonc v0.10.7+bc794d341. That compiler crashes with Moonc.Hashed_type.Unresolved_type_variable while compiling moonbitlang/x@0.4.43, then emits 242 cascading parse/type errors before the repository code is tested.

Validation

• reproduced the failure with an isolated download of the current latest toolchain
• downloaded the immutable 0.10.6+80dc50f24 archive into a separate isolated directory and passed moon check --target js modules/bit
• pkf run test-js-build: 41/41 passed
• pkf run check
• actionlint -shellcheck= .github/workflows/*.yml
• parsed all workflow and composite-action YAML files
• git diff --check