various: add golangci-lint, fix issues (#7905)

This adds an initial and intentionally minimal configuration for
golang-ci, fixes the issues reported, and adds a GitHub Action to check
new pull requests against this linter configuration.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I8f38fbc315836a19a094d0d3e986758b9313f163

client/tailscale/acl.go

@@ -436,7 +436,7 @@ func (c *Client) ValidateACLJSON(ctx context.Context, source, dest string) (test
 		}
 	}()
 
-	tests := []ACLTest{ACLTest{User: source, Allow: []string{dest}}}
+	tests := []ACLTest{{User: source, Allow: []string{dest}}}
 	postData, err := json.Marshal(tests)
 	if err != nil {
 		return nil, err