Use latest nightly Rust toolchain for riscv example. (#26)

This lets the main crate use some newer features.

examples/riscv/Cargo.toml

@@ -9,7 +9,7 @@ edition = "2018"
 [dependencies]
 log = "0.4"
 riscv = "0.6"
-opensbi-rt = { git = "https://github.com/rcore-os/opensbi-rt.git", rev = "1308cc5" }
+opensbi-rt = { git = "https://github.com/rcore-os/opensbi-rt.git", rev = "abdfeb72" }
 fdt = "0.1.4"
 virtio-drivers = { path = "../.." }
 lazy_static = { version = "1.4", features = ["spin_no_std"] }

examples/riscv/rust-toolchain

@@ -1 +1 @@
-nightly-2021-03-01
+nightly-2022-11-03