Add build step to compile sandboxes on iOS to reduce the risk of committing sandbox changes that are not compiling.
<rdar://problem/116677504>
Pull request: https://github.com/WebKit/WebKit/pull/18838
Committed 269317@main (e5d7b0cb476d): <https://commits.webkit.org/269317@main> Reviewed commits have been landed. Closing PR #18838 and removing active labels.