feat: add --apply-env-vars-to-build flag to push (#734)

[MatousMarik]

Part of #734 (the applyEnvVarsToBuild toggle half).

What

• New --apply-env-vars-to-build boolean flag on apify push — sets applyEnvVarsToBuild: true on the Actor version, so env vars from actor.json are also available during the Docker build.
• When the flag is omitted, the field is left out of the API payload entirely, so a value previously set in Console is preserved (pushes to existing Actors don't reset it).
• Regenerated docs/ via pnpm run update-docs.

Why

Env vars synced by apify push currently apply only at runtime; making them apply to the build requires flipping a switch in Console. That breaks CI and fresh pushes of Actors that need build-time secrets (e.g. a GitHub token as a Docker build arg).

Tests

• New [api] test in test/api/commands/push.test.ts: push with the flag → version has applyEnvVarsToBuild: true; push again without it → value preserved. Full push API suite passes (13/13), plus test:local, lint, format, build.

Follow-ups (separate PRs)

• applyEnvVarsToBuild field in .actor/actor.json (persistent per-repo, incl. explicit false to turn it off).
• --env KEY=VALUE passing on push (the other half of Pass environment variables to apify push and toggle applyEnvVarsToBuild #734).

No new dependencies; no install-size impact.