While running various tests for #28387, I keep ending up with an
unwritable GOROOT after a failure.

While the unwritable GOROOT is a fairly exotic condition (normally
only happens on builders), it's somewhat annoying when debugging, so
I'm switching all of the log.Fatal* call sites to use the existing
fatalf function, which supports general atexit-like cleanup.

Updates #28387

Change-Id: I473cda7eacd9ad82bdeab647766373126dc7390e
Reviewed-on: https://go-review.googlesource.com/c/go/+/207341
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>