cmd/go: use the correct gofmt binary in TestScript/fmt_load_errors
cmd/go: use the correct gofmt binary in TestScript/fmt_load_errors

Otherwise, this test may fail when GOROOT/bin is not in $PATH.

Change-Id: Id744f365f6604716207184c9ea49436ab2e3a835
Reviewed-on: https://go-review.googlesource.com/c/go/+/391802
Trust: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,CAFxX/go,golang/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go