cmd/trace: fix static file reference

Use runtime.GOROOT instead of os.Getenv("GOROOT") to reference
trace-viewer html file. GOROOT env var is not necessary set,
runtime.GOROOT has a default value for such case.

Change-Id: I906a720f6822915bd9575756e6cbf6d622857c2b
Reviewed-on: https://go-review.googlesource.com/13593Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/trace/trace.go

@@ -10,8 +10,8 @@ import (
 	"internal/trace"
 	"log"
 	"net/http"
-	"os"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 )
@@ -63,7 +63,7 @@ var templTrace = `
 // httpTraceViewerHTML serves static part of trace-viewer.
 // This URL is queried from templTrace HTML.
 func httpTraceViewerHTML(w http.ResponseWriter, r *http.Request) {
-	http.ServeFile(w, r, filepath.Join(os.Getenv("GOROOT"), "misc", "trace", "trace_viewer_lean.html"))
+	http.ServeFile(w, r, filepath.Join(runtime.GOROOT(), "misc", "trace", "trace_viewer_lean.html"))
 }
 
 // httpJsonTrace serves json trace, requested from within templTrace HTML.