pin nightly version in CI

rust-lang/rust#97791 appears to have broken -Z build-std (?)

Author: japaric

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
           override: true
           profile: minimal
           target: thumbv7m-none-eabi
-          toolchain: nightly
+          toolchain: nightly-2022-06-18
       - run: cargo install --path . --debug
       - name: Can analyze example firmware
         run: cargo test