Don't use nightly for CI

Everything now builds on stable. Hopefully nightly doesn't break
anything (that's what crater's for, right?)

testing/ci.sh

@@ -18,7 +18,6 @@ export RUSTDOCFLAGS='-D warnings'
 # dependencies
 which cargo-bloat > /dev/null || cargo install cargo-bloat
 if [ -z "$OFFLINE" ]; then
-    rustup toolchain add nightly
     (
     cd embassy/demos/picow
     rustup target add thumbv6m-none-eabi
@@ -32,10 +31,6 @@ cargo build
 cargo doc
 cargo test --doc
 
-# nightly
-cargo +nightly build
-cargo +nightly test
-
 (
 cd async
 # only test lib since some examples are broken