chore: use e2e updater url for e2e builds

[piotr-iohk]

This PR points E2E builds to a separate updater release.json hosted in bitkit-e2e-tests, so production critical updates don't block E2E test runs.

Description

The production updater currently has critical: true for both platforms. E2E builds from master with lower build numbers hit the blocking CriticalUpdate screen immediately, making all tests fail.

When isE2eTest is true, RELEASE_URL now resolves to:
https://github.com/synonymdev/bitkit-e2e-tests/releases/download/updater/release.json

That file has buildNumber: 0, critical: false by default, so E2E builds are never blocked. When update flow testing is needed, the E2E release.json can be updated independently.

Preview

N/A

QA Notes

1. Build with E2E=true and verify the app does not show a critical update screen on startup
2. Build without E2E=true and verify the production updater still works as before

Made with Cursor

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[ovitrif]

lgtm