Run gofmt on all files, complain about the diff. (#106)

.github/workflows/build-and-test.yaml

@@ -8,6 +8,7 @@ jobs:
       - uses: actions/setup-go@v2
         with:
           go-version: 1.17
+      - run: DIFF="$( find . -name '*.go' -print0 | xargs -0 gofmt -l )"; if [ -n "$DIFF" ]; then echo "Files not formatted, run gofmt:"; echo "$DIFF"; exit 1; fi
       - run: sudo apt-get install libusb-1.0-0-dev
       - run: go install golang.org/x/tools/cmd/cover@latest
       - run: go install golang.org/x/lint/golint@latest