cmd/dist: skip rebuild before running tests when on the build systems

Updates #24300

Change-Id: I7752dab67e15a6dfe5fffe5b5ccbf3373bbc2c13
Reviewed-on: https://go-review.googlesource.com/99296Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/test.go

@@ -133,7 +133,11 @@ func (t *tester) run() {
 	// to break if we don't automatically refresh things here.
 	// Rebuilding is a shortened bootstrap.
 	// See cmdbootstrap for a description of the overall process.
-	if !t.listMode {
+	//
+	// But don't do this if we're running in the Go build system,
+	// where cmd/dist is invoked many times. This just slows that
+	// down (Issue 24300).
+	if !t.listMode && os.Getenv("GO_BUILDER_NAME") == "" {
 		goInstall("go", append([]string{"-i"}, toolchain...)...)
 		goInstall("go", append([]string{"-i"}, toolchain...)...)
 		goInstall("go", "std", "cmd")