builder: exclude namespace mounts from Cargo clean

host/src/builder/mod.rs

@@ -95,13 +95,15 @@ impl Builder {
         // Cargo clean must remove its directory, not a namespace mount point.
         let target_mount = base.join("target");
         let target = target_mount.join("cargo");
+        // Keep /tmp's namespace mountpoints outside the directory Cargo clean removes.
+        let temporary = target_mount.join("tmp");
         let registry = base.join("cargo");
         let cache = base.join("kache");
         if source.exists() {
             std::fs::remove_dir_all(&source)?;
         }
         let before = files::copy_out(self.root.as_ref(), &workspace, &source).await?;
-        for path in [&target, &registry, &cache, &target.join("tmp")] {
+        for path in [&target, &registry, &cache, &temporary] {
             std::fs::create_dir_all(path)?;
         }
         std::fs::write(
@@ -187,7 +189,7 @@ impl Builder {
                 "/nix/store",
             ])
             .arg("--bind")
-            .arg(target.join("tmp"))
+            .arg(&temporary)
             .arg("/tmp")
             .arg("--ro-bind")
             .arg(executable("sh")?)