Require nightly toolchain for Rust

jwnrt · jwnrt · commit 4a27f7d1a0b7 · 2024-08-23T11:13:27.000+01:00
We're using the (long-term) unstable `build-std` feature which requires
the nightly compiler.
diff --git a/sw/rust/rust-toolchain.toml b/sw/rust/rust-toolchain.toml
@@ -1,2 +1,3 @@
 [toolchain]
 targets = ["riscv32imc-unknown-none-elf"]
+channel = "nightly"

Original file line number	Diff line number	Diff line change
`@@ -1,2 +1,3 @@`
`1`	`1`	`[toolchain]`
`2`	`2`	`targets = ["riscv32imc-unknown-none-elf"]`
	`3`	`+channel = "nightly"`