cmd/dist: report correct CC name in error about missing CC

Fixes #25611

Change-Id: I487463584e4d9d99cf5d8db0c9a4b4e66464ecd8
Reviewed-on: https://go-review.googlesource.com/114935
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/build.go

@@ -1438,7 +1438,7 @@ func checkCC() {
 		fatalf("cannot invoke C compiler %q: %v\n\n"+
 			"Go needs a system C compiler for use with cgo.\n"+
 			"To set a C compiler, set CC=the-compiler.\n"+
-			"To disable cgo, set CGO_ENABLED=0.\n%s%s", defaultcc, err, outputHdr, output)
+			"To disable cgo, set CGO_ENABLED=0.\n%s%s", defaultcc[""], err, outputHdr, output)
 	}
 }