build: do not reset DOTEST_EXTRA

We were clobbering the existing flags when appending breaking the staging route.

Author: compnerd

utils/build-script-impl

@@ -3091,8 +3091,8 @@ for host in "${ALL_HOSTS[@]}"; do
                     LIBDISPATCH_BUILD_DIR="$(build_directory ${host} libdispatch)"
                     FOUNDATION_BUILD_DIR=$(build_directory ${host} foundation)
                     DOTEST_EXTRA="-I${FOUNDATION_BUILD_DIR}"
-                    DOTEST_EXTRA="-Xcc -F${FOUNDATION_BUILD_DIR}"
-                    DOTEST_EXTRA="-Xcc -F${FOUNDATION_BUILD_DIR}/CoreFoundation-prefix/System/Library/Frameworks"
+                    DOTEST_EXTRA="${DOTEST_EXTRA} -Xcc -F${FOUNDATION_BUILD_DIR}"
+                    DOTEST_EXTRA="${DOTEST_EXTRA} -Xcc -F${FOUNDATION_BUILD_DIR}/CoreFoundation-prefix/System/Library/Frameworks"
                     DOTEST_EXTRA="${DOTEST_EXTRA} -I${FOUNDATION_BUILD_DIR}/swift"
                     DOTEST_EXTRA="${DOTEST_EXTRA} -I${LIBDISPATCH_SOURCE_DIR}"
                     DOTEST_EXTRA="${DOTEST_EXTRA} -L${FOUNDATION_BUILD_DIR}"