Switch format check to gofmt directly (#100)

The `goimport` tool cannot cope with the `golang.org/x/tools` package: if you add the checksum it says you need via `go mod download` it then complains you need to `go mod tidy`, which then removes that checksum.

```
go: downloading golang.org/x/tools v0.0.0-20201201161351-ac6f37ff4c2a
go: updates to go.mod needed; to update it:
	go mod tidy
```
vs
```
missing go.sum entry for module providing package golang.org/x/tools/cmd/goimports; to add:
	go mod download golang.org/x/tools
```

.github/workflows/test.yaml

@@ -14,16 +14,16 @@ jobs:
           key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
           restore-keys: |
             ${{ runner.os }}-go-
-
+            
       - name: Check formatting
         run: |
-          files=$(go run golang.org/x/tools/cmd/goimports -l -local "github.com/G-Research" .)
+          files=$(gofmt -l .)
           if [[ $files != "" ]]; then
-            echo "Files need goimports running on them:"
+            echo "Files need gofmt run on them:"
             echo "$files"
             exit 1
           fi
-
+          
       - name: Build binary
         run: |
           go build ./cmd/...
@@ -34,4 +34,4 @@ jobs:
 
       - name: Run tests
         run: |
-          go test ./...
+          go test ./...