Install the correct no_std target in CI

Author: wetheredge

.github/workflows/test.yml

@@ -76,7 +76,7 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
-      - run: rustup target add thumbv6m-none-eabi
+      - run: rustup target add thumbv7em-none-eabihf
       - run: cargo check --all-targets --no-default-features
       - run: cargo check --lib --target thumbv7em-none-eabihf --no-default-features -F use-rstar_0_9,serde
       - run: cargo test --all-features