cmd/dist: more informative error for mkdtemp failing

The Darwin builders are all failing here but strerror doesn't provide context.

R=golang-dev, bradfitz, adg
CC=golang-dev
https://golang.org/cl/13095043

src/cmd/dist/unix.c

@@ -467,8 +467,9 @@ xworkdir(void)
 	if(b.len == 0)
 		bwritestr(&b, "/var/tmp");
 	bwritestr(&b, "/go-cbuild-XXXXXX");
-	if(mkdtemp(bstr(&b)) == nil)
-		fatal("mkdtemp: %s", strerror(errno));
+	p = bstr(&b);
+	if(mkdtemp(p) == nil)
+		fatal("mkdtemp(%s): %s", p, strerror(errno));
 	p = btake(&b);
 
 	bfree(&b);