.github,cmd/cigocacher: add flags --version --stats --cigocached-host

Add flags:

* --cigocached-host to support alternative host resolution in other
  environments, like the corp repo.
* --stats to reduce the amount of bash script we need.
* --version to support a caching tool/cigocacher script that will
  download from GitHub releases.

Updates tailscale/corp#10808

Change-Id: Ib2447bc5f79058669a70f2c49cef6aedd7afc049
Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

cmd/cigocacher/http.go

@@ -32,12 +32,6 @@ func tryReadErrorMessage(res *http.Response) []byte {
 }
 
 func (c *gocachedClient) get(ctx context.Context, actionID string) (outputID string, resp *http.Response, err error) {
-	// TODO(tomhjp): make sure we timeout if cigocached disappears, but for some
-	// reason, this seemed to tank network performance.
-	// // Set a generous upper limit on the time we'll wait for a response. We'll
-	// // shorten this deadline later once we know the content length.
-	// ctx, cancel := context.WithTimeout(ctx, time.Minute)
-	// defer cancel()
 	req, _ := http.NewRequestWithContext(ctx, "GET", c.baseURL+"/action/"+actionID, nil)
 	req.Header.Set("Want-Object", "1") // opt in to single roundtrip protocol
 	if c.accessToken != "" {