cmd/golangorg: fix CachedURL, update rebuild template

The "not modified" response code is 304, not 206. Oops.
Use named constants to avoid similar mistakes in the future.

Also update rebuild template to show more version information.

For golang/go#57120.
For golang/go#58884.

Change-Id: I2c3ddf25cede0b5a853fa971226463a997f168c7
Reviewed-on: https://go-review.googlesource.com/c/website/+/515455
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
diff --git a/_content/rebuild.html b/_content/rebuild.html
index 8931be0..cf59587 100644
--- a/_content/rebuild.html
+++ b/_content/rebuild.html
@@ -55,8 +55,9 @@
 {{$Report := json gorebuild}}
 {{with $Report}}
 
-Using gorebuild from {{.Version}}<br><br>
-
+Gorebuild version {{.Version}}.<br>
+Built with Go version {{.GoVersion}}, {{.GOOS}}-{{.GOARCH}}.<br>
+<br>
 Rebuild started at {{(rfc3339 .Start).UTC.Format "2006-01-02 15:04:05"}} UTC.<br>
 Rebuild finished at {{(rfc3339 .End).UTC.Format "2006-01-02 15:04:05"}} UTC.<br>
 Elapsed time: {{((rfc3339 .End).Sub (rfc3339 .Start)).Round 1e9}}.
diff --git a/cmd/golangorg/server.go b/cmd/golangorg/server.go
index 2a7f66b..7665efa 100644
--- a/cmd/golangorg/server.go
+++ b/cmd/golangorg/server.go
@@ -855,13 +855,13 @@
 		return nil, fmt.Errorf("loading rebuild report JSON: %v", err)
 	}
 	defer resp.Body.Close()
-	if resp.StatusCode == 206 {
+	if resp.StatusCode == http.StatusNotModified {
 		// Unmodified.
 		log.Printf("checked %s - unmodified", c.url)
 		return c.data, c.err
 	}
 	log.Printf("reloading %s", c.url)
-	if resp.StatusCode != 200 {
+	if resp.StatusCode != http.StatusOK {
 		return nil, fmt.Errorf("loading rebuild report JSON: %v", resp.Status)
 	}
 	c.etag = resp.Header.Get("Etag")