Create $XDG_RUNTIME_DIR in /run instead

Using tempfile::Builder::tempdir() defaults to creating this directory
in /tmp, which is shared with the host and often somewhat persistent.
This causes one extra directory to be created in the host /tmp every
time muvm runs.

Create this directory in /run instead, which is non-shared, and a more
idiomatic place for $XDG_RUNTIME_DIR.

Signed-off-by: Vivian Wang <[email protected]>

Author: dramforever

crates/muvm/src/guest/user.rs

@@ -17,7 +17,7 @@ pub fn setup_user(uid: Uid, gid: Gid) -> Result<PathBuf> {
     let path = tempfile::Builder::new()
         .prefix(&format!("muvm-run-{uid}-"))
         .permissions(Permissions::from_mode(0o700))
-        .tempdir()
+        .tempdir_in("/run")
         .context("Failed to create temp dir for `XDG_RUNTIME_DIR`")?
         .into_path();
     // SAFETY: Safe if and only if `muvm-guest` program is not multithreaded.