feat: support using Bicep from PATH

[nanookclaw]

Summary

• add an optional use-bicep-from-path action input
• when enabled, compile Bicep files using an executable already available on PATH
• preserve the existing cache/download behavior when the input is omitted or when no PATH executable is found

Closes #166

Testing

• npm ci (completed with EBADENGINE warning because this environment uses Node v22.22.0 while the repo declares >=24; install still succeeded)
• npm run build:packages
• npm run typecheck
• npm test -- --run packages/bicep-deploy-common/test/config.test.ts packages/bicep-deploy-common/test/cache.test.ts
• npm run build
• npm test
• npm run lint
• node --check dist/index.cjs && node --check dist/index.mjs
• git diff --check