Build: Only install Playwright dependencies when needed

Build: Only install Playwright dependencies when needed

PR gh-5190 added support for running tests on Playwright WebKit
in CI. For efficiency reasons, Playwright dependencies are only
installed for the `test:browser` npm script. However, that same
script is also used for Firefox ESR testing.

This change makes Playwright dependencies installed only for cases
where `WebKitHeadless` exists on the list of tested browsers.

Closes gh-5204
Ref gh-5190