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

Author: joker-eph

.ci/monolithic-linux.sh

@@ -54,4 +54,4 @@ cmake -S ${MONOREPO_ROOT}/llvm -B ${BUILD_DIR} \
 
 echo "--- ninja"
 # Targets are not escaped as they are passed as separate arguments.
-ninja -C "${BUILD_DIR}" ${targets}
+ninja -C -k 0 "${BUILD_DIR}" ${targets}

.ci/monolithic-windows.sh

@@ -62,4 +62,4 @@ cmake -S ${MONOREPO_ROOT}/llvm -B ${BUILD_DIR} \
 
 echo "--- ninja"
 # Targets are not escaped as they are passed as separate arguments.
-ninja -C "${BUILD_DIR}" ${targets}
+ninja -C -k 0 "${BUILD_DIR}" ${targets}