chore: a separate target directory for runtimetest

building static-linked runtimetest requires additional RUSTFLAGS env
var. Unfortunately, the additional env var invalids the build cache,
leading cargo rebuilding everytime.

See rust-lang/cargo#8716

A temporary solution is to build runtimetest in a seperate
`runtimetest-target` directory.

Signed-off-by: Tony Duan <tony84727@gmail.com>

.gitignore

@@ -2,6 +2,7 @@
 .idea/
 
 **/target
+/runtimetest-target
 .vagrant/
 
 tags

scripts/build.sh

@@ -36,7 +36,7 @@ OUTPUT=${output:-$ROOT/bin}
 
 cargo build --target ${TARGET} ${OPTION} --bin youki
 cargo build --target ${TARGET} ${OPTION} --bin integration_test
-RUSTFLAGS="-Ctarget-feature=+crt-static" cargo build --target ${TARGET} ${OPTION} --bin runtimetest
+CARGO_TARGET_DIR=${ROOT}/runtimetest-target RUSTFLAGS="-Ctarget-feature=+crt-static" cargo build --target ${TARGET} ${OPTION} --bin runtimetest
 
 mv ${ROOT}/target/${TARGET}/${VERSION}/{youki,integration_test,runtimetest} ${OUTPUT}/