bwrap: use fusermount3 instead of fusermount

rofiles-fuse is now built against libfuse3 on all the platforms we care
about in rpm-ostree. Accordingly also switch over the fusermount utility
we use to the v3 version to match. This also allows composes to drop out libfuse2 if
nothing else pulls it in.

See also: coreos#5047 (comment)

rust/src/bwrap.rs

@@ -122,7 +122,7 @@ impl Drop for RoFilesMount {
             return;
         };
         // We need to unmount before letting the tempdir cleanup run.
-        let success = Command::new("fusermount")
+        let success = Command::new("fusermount3")
             .arg("-u")
             .arg(tempdir.path())
             .status()