scripts: propagate RUSTFLAGS and TARGET_CC to test scripts

There is no need to set them in the test scripts while the main script
already has them.

The consolidates how things are done.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

scripts/run_integration_tests_live_migration.sh

@@ -59,9 +59,7 @@ if [ ! -f "$VMLINUX_IMAGE" ]; then
 fi
 
 CFLAGS=""
-TARGET_CC=""
 if [[ "${BUILD_TARGET}" == "x86_64-unknown-linux-musl" ]]; then
-    TARGET_CC="musl-gcc"
     CFLAGS="-I /usr/include/x86_64-linux-musl/ -idirafter /usr/include/"
 fi