build: in clean.bash, look for 'go' in GOROOT/bin instead of GOBIN

Updates #14340
Updates #32674
Fixes #36659

Change-Id: I5bfaba4e53dab894d113dd5065794d66f1a25f2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/215478
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/clean.bash

@@ -11,7 +11,7 @@ if [ ! -f run.bash ]; then
 fi
 export GOROOT="$(cd .. && pwd)"
 
-gobin="${GOBIN:-../bin}"
+gobin="${GOROOT}"/bin
 if ! "$gobin"/go help >/dev/null 2>&1; then
 	echo 'cannot find go command; nothing to clean' >&2
 	exit 1