initfs: Use CARGO_TARGET_TMPDIR in test

initfs/.gitignore

@@ -1,4 +0,0 @@
-Cargo.lock
-# TODO: Maybe use an OUT_DIR environment variable, or mktemp, rather than
-# writing directly to the git repository root?
-/out.img

initfs/tools/tests/archive_and_read.rs

@@ -75,7 +75,7 @@ fn archive_and_read() -> Result<()> {
     env_logger::init();
 
     let args = redox_initfs_tools::Args {
-        destination_path: Path::new("out.img"),
+        destination_path: &Path::new(env!("CARGO_TARGET_TMPDIR")).join("out.img"),
         source: Path::new("data"),
         bootstrap_code: None,
         max_size: redox_initfs_tools::DEFAULT_MAX_SIZE,