Because of another cmake footgun.
Pull request: https://github.com/WebKit/WebKit/pull/9912
Actually, re-titling bug. We can't enable debug fission on sccache clang builds yet.
Committed 260444@main (55c9be47c76e): <https://commits.webkit.org/260444@main> Reviewed commits have been landed. Closing PR #9912 and removing active labels.