Add Dutch locales to browser images

[tnsardesai]

Summary

• Generate nl_NL.UTF-8 in headful and headless browser images.
• Copy Chrome for Testing's Dutch nl.pak to nl-NL.pak so the regional locale does not fall back to generic nl for Intl formatting.
• Existing tzdata supplies Europe/Amsterdam; no additional desktop language package is needed.

Validation

• go test ./cmd/chromium-launcher — passed.
• Downloaded the exact pinned Chrome for Testing 152.0.7977.42 archive and verified it includes nl.pak.
• Built a minimal Ubuntu 22.04 validation container with the same locale-generation command, pinned browser, and resource alias. Both headless and Xvfb headful probes returned navigator.language=nl-NL, Intl locale nl-NL, and timezone Europe/Amsterdam; locale charmap returned UTF-8.
• Removing nl-NL.pak reproduced Intl fallback to nl.
• Complete production image builds and server e2e tests were not run. No image release or deployment was performed.