build: checks.yml GitHub Actions workflow doesn't need goimports installed

The importscheck script now uses "go run" to run this program and so
the Go toolchain will install it automatically using the version number
of golang.org/x/tools specified in our go.mod file.

.github/workflows/checks.yml

@@ -180,10 +180,6 @@ jobs:
           restore-keys: |
             protobuf-tools-
 
-      - name: Install CI tooling
-        run: |
-          go install golang.org/x/tools/cmd/goimports@v0.1.11
-
       - name: "Code consistency checks"
         run: |
           make fmtcheck importscheck generate staticcheck exhaustive protobuf