[CI] Use env variable to enable pip breaking system packages

This patch uses an env variable instead of the --break-system-packages
flag. This enables the heterogenous configuration between the old and
new premerge systems as the old premerge container does not recognize
the --break-system-packages flag. An env variable will work on new
premerge and have no impact on old premerge.

Author: boomanaiden154

.ci/monolithic-linux.sh

@@ -53,9 +53,10 @@ targets="${2}"
 lit_args="-v --xunit-xml-output ${BUILD_DIR}/test-results.xml --use-unique-output-file-name --timeout=1200 --time-tests"
 
 echo "--- cmake"
-pip install --break-system-packages -q -r "${MONOREPO_ROOT}"/mlir/python/requirements.txt
-pip install --break-system-packages -q -r "${MONOREPO_ROOT}"/lldb/test/requirements.txt
-pip install --break-system-packages -q -r "${MONOREPO_ROOT}"/.ci/requirements.txt
+export PIP_BREAK_SYSTEM_PACKAGES=1
+pip install -q -r "${MONOREPO_ROOT}"/mlir/python/requirements.txt
+pip install -q -r "${MONOREPO_ROOT}"/lldb/test/requirements.txt
+pip install -q -r "${MONOREPO_ROOT}"/.ci/requirements.txt
 cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
       -D LLVM_ENABLE_PROJECTS="${projects}" \
       -G Ninja \