build: fail nicely if somebody runs all.bash from a binary tarball package

Fixes golang/go#20008.

Change-Id: I7a429490320595fc558a8c5e260ec41bc3a788e2
Reviewed-on: https://go-review.googlesource.com/41858Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/all.bash

@@ -8,6 +8,10 @@ if [ ! -f make.bash ]; then
 	echo 'all.bash must be run from $GOROOT/src' 1>&2
 	exit 1
 fi
+if [ ! -f cmd/api/run.go ]; then
+	echo 'all.bash cannot be run from a binary download package' 1>&2
+	exit 1
+fi
 OLDPATH="$PATH"
 . ./make.bash "$@" --no-banner
 bash run.bash --no-rebuild