Update picow to new cyw43

Fixes build failures. Toolchain is updated, but we can't upgrade to
latest nightly because it fails with an ICE, possibly
rust-lang/rust#110206

thread 'rustc' panicked at 'DefId(35:311 ~ embedded_io[99f8]::asynch::Write::write::{opaque#0}) does not have a "def_span"', compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs:205:1

Author: mkj

async/rust-toolchain.toml

@@ -1,7 +1,9 @@
 # Before upgrading check that everything is available on all tier1 targets here:
 # https://rust-lang.github.io/rustup-components-history
+
+# 2023-04-15 has ICE building picow demo
 [toolchain]
-channel = "nightly-2023-04-07"
+channel = "nightly-2023-04-08"
 components = [ "rust-src", "rustfmt" ]
 targets = [
     "thumbv6m-none-eabi",