env: fix handling of long lowerdir paths in newer kernel/mount versions

The new "lowerdir+" option can be used to specify multiple layers
when mounting. This avoids a limitation with the "lowerdir" option which
is limited to a max of 256 characters.

Fixes: #968

crates/spfs/src/env.rs

@@ -1126,11 +1126,12 @@ pub(crate) fn get_overlay_args<P: AsRef<Path>>(
     // the rightmost on the command line is the bottom layer, and the
     // leftmost is on the top). For more details see:
     // https://docs.kernel.org/filesystems/overlayfs.html#multiple-lower-layers
-    args.push_str("lowerdir=");
     for path in layer_dirs.iter().rev() {
+        args.push_str("lowerdir+=");
         args.push_str(&path.as_ref().to_string_lossy());
-        args.push(':');
+        args.push(',');
     }
+    args.push_str("lowerdir+=");
     args.push_str(&rt.config.lower_dir.to_string_lossy());
 
     args.push_str(",upperdir=");