fix: Set GOBIN environment variable for apidiff installation

Author: yongruilin

hack/apidiff.sh

@@ -55,10 +55,19 @@ 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
 
+# Ensure the remote is set
+if ! git remote get-url origin &> /dev/null; then
+    git remote add origin https://github.com/kubernetes-sigs/structured-merge-diff.git
+fi
+
+# Fetch the latest changes from the remote
+git fetch origin
+
 # Fetch common base if -r is not set
 if [ -z "${REFERENCE_REVISION}" ]; then
     echo "Determining common base with origin/master..."