Run pre-merge build with -k 0 to ensure all tests runs (#84828)

The -k option allows to continue the build after failures as much as
possible. This is useful here because when we run

> ninja check-llvm check-clang

we would like the clang tests to run even if there is a failure in a
llvm tests.

The downside is that a build failure in one file that would prevent from
running any test does not prevent from building more targets, wasting
build resources potentially.

Fixes #83371