Remove toolchain parameter for workflow

It seems the github action will pick up the rust-toolchain file and if
we specify it as an input, it will have a higher priority

.github/workflows/cargo_check.yml

@@ -19,9 +19,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
           target: thumbv7em-none-eabi
-          override: true
       - uses: actions/setup-python@v1
         with:
           python-version: 3.7