cmd/dist: use "set" instead of "export" in diagnostic message

On Windows, "export" doesn't mean anything, but Windows users are the
most likely to see this message.

Fixes #15977

Change-Id: Ib09ca08a7580713cacb65d0cdc43730765c377a8
Reviewed-on: https://go-review.googlesource.com/23840Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/build.go

@@ -1143,8 +1143,8 @@ func checkCC() {
 		}
 		fatal("cannot invoke C compiler %q: %v\n\n"+
 			"Go needs a system C compiler for use with cgo.\n"+
-			"To set a C compiler, export CC=the-compiler.\n"+
-			"To disable cgo, export CGO_ENABLED=0.\n%s%s", defaultcc, err, outputHdr, output)
+			"To set a C compiler, set CC=the-compiler.\n"+
+			"To disable cgo, set CGO_ENABLED=0.\n%s%s", defaultcc, err, outputHdr, output)
 	}
 }