chore(web-tests): use is-ci instead of shell

[colinaaa]

Summary

Fix Rspack EcoSystem CI failure (since CI could be something like true or github).

This would also make web-tests:build work on Windows.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[codecov]

Bundle Report

Changes will increase total bundle size by 3.35kB (0.52%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
@lynx-js/example-react-lynx-cjs	321.14kB	3.35kB (1.05%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: @lynx-js/example-react-lynx-cjs

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
main.lynx.bundle	1.74kB	89.87kB	1.97%
.rspeedy/main/main-thread.js	744 bytes	40.8kB	1.86%
.rspeedy/main/background.*.js	864 bytes	39.91kB	2.21%

[codspeed-hq]

CodSpeed Performance Report

Merging #784 will not alter performance

_{Comparing colinaaa:colin/0513/is-ci (75490a3) with main (a80c6f4)}

Summary

✅ 2 untouched benchmarks