GitHub CI: drop use of 'dep' and fully use Go modules

Author: dolmen

.github/workflows/go.yml

@@ -23,12 +23,7 @@ jobs:
       id: go
 
     - name: Get dependencies
-      run: |
-        go get -v -t -d ./...
-        if [ -f Gopkg.toml ]; then
-            curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
-            dep ensure
-        fi
+      run: go mod download
 
     - name: Build
       run: go build -v .