fix: Set GOBIN environment variable for apidiff installation

Author: yongruilin

Diff for: hack/apidiff.sh

@@ -55,6 +55,7 @@ fi
 
 # Check for apidiff tool, install it if not found
 if ! command -v "${API_DIFF_TOOL}" &> /dev/null; then
+    GOBIN=${GOBIN:-$(go env GOPATH)/bin}
     echo "Installing apidiff into ${GOBIN}."
     go install golang.org/x/exp/cmd/apidiff@latest
 fi