Improve CI

* Don't run the CI twice in pull requests
* Add check for `go.mod` consistency
* Upgrade Go version to `1.16.x`
* Ugrade the Go `actions/setup-go` actions library to `v2`

.github/workflows/go.yml

@@ -1,24 +1,40 @@
 name: build
-on: [push, pull_request]
-jobs:
+on:
+  push:
+    branches:
+      - master
+      - main
+  pull_request:
 
-  test-build:
+jobs:
+  test:
     name: Test & Build
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.15
-      uses: actions/setup-go@v1
+    - name: Set up Go 1.16
+      uses: actions/setup-go@v2
       with:
-        go-version: 1.15.3
-      id: go
+        go-version: ^1.16
 
     - name: Check out code into the Go module directory
       uses: actions/checkout@v2
+
+    - name: Run go mod tidy
+      run: |
+        set -e
+        go mod tidy
+        output=$(git status -s)
+        if [ -z "${output}" ]; then
+         exit 0
+        fi
+        echo 'We wish to maintain a tidy state for go mod. Please run `go mod tidy` on your branch, commit and push again.'
+        echo 'Running `go mod tidy` on this CI test yields with the following changes:'
+        echo "$output"
+        exit 1
       
     - name: Test
-      run: |
-        go test -race ./...
+      run: go test -race ./...
 
     - name: Build
       run: go build  ./...