Fix compiling with CARGO_TARGET_DIR

Previously it would error:

```
    Finished dev [unoptimized + debuginfo] target(s) in 26.93s
mv target/debug/main raytracer_cg_llvm
mv: cannot stat 'target/debug/main': No such file or directory
```

Author: jyn514

build.sh

@@ -26,6 +26,7 @@ while [[ $# != 0 ]]; do
 done
 
 # Build cg_clif
+unset CARGO_TARGET_DIR
 export RUSTFLAGS="-Zrun_dsymutil=no"
 if [[ "$CHANNEL" == "release" ]]; then
     cargo build --release

prepare.sh

@@ -24,6 +24,7 @@ git checkout -- .
 git checkout 804a7a21b9e673a482797aa289a18ed480e4d813
 
 # build with cg_llvm for perf comparison
+unset CARGO_TARGET_DIR
 cargo build
 mv target/debug/main raytracer_cg_llvm
 popd