ci: Install latest Rust

The ci of docker-smoke gets errors otherwise:

````
error[E0658]: deriving `Default` on enums is experimental
   --> src/passthrough/mod.rs:345:17
    |
345 | #[derive(Debug, Default, Clone, Eq, PartialEq)]
    |                 ^^^^^^^
    |
    = note: see issue #86985 <rust-lang/rust#86985> for more information
    = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0658`.
error: could not compile `fuse-backend-rs` due to previous error
make: *** [Makefile:4: build] Error 101
make: *** [Makefile:44: docker-smoke] Error 2
```

Author: matthiasgoergens

Makefile

@@ -41,4 +41,4 @@ smoke-macos: check-macos
 	cargo test --features="fusedev" -- --nocapture
 
 docker-smoke:
-	docker run --env RUST_BACKTRACE=1 --rm --privileged -v ${current_dir}:/fuse-rs rust:1.59 sh -c "rustup component add clippy rustfmt; cd /fuse-rs; make smoke-all"
+	docker run --env RUST_BACKTRACE=1 --rm --privileged -v ${current_dir}:/fuse-rs rust:1.68 sh -c "rustup component add clippy rustfmt; cd /fuse-rs; make smoke-all"