fix(utils): make install-kubectl.sh work on macOS (#976)

install-kubectl.sh always downloaded the linux/amd64 kubectl, so on macOS
it installed an unexecutable Linux ELF (later kubectl calls fail with an
exec format error). Detect OS (uname -s) and arch (uname -m) and download
the matching bin/{linux,darwin}/{amd64,arm64}/kubectl, mirroring the OS/arch
detection added to install-minikube-cluster.sh in #970.

Also: treat an existing kubectl as installed only if it actually runs
(`kubectl version --client`), so a stale Linux binary left on PATH on macOS
is replaced rather than skipped; and use `curl -f` so HTTP errors abort
instead of saving an error page as the binary.

Follow-up to the review on #970; relates to #931.

Signed-off-by: HumphreySun98 <humphreysun98@gmail.com>
Co-authored-by: Rui Zhang <51696593+ruizhang0101@users.noreply.github.com>

Author: HumphreySun98

utils/install-kubectl.sh

@@ -5,11 +5,30 @@ set -e
 KUBECTL_DIR="$HOME/.local/bin"
 KUBECTL_PATH="$KUBECTL_DIR/kubectl"
 
+# Detect host OS and architecture so we download the matching kubectl build.
+# Without this the script always pulled the linux/amd64 binary, which on macOS
+# installs an unexecutable Linux ELF (exec format error on later kubectl calls).
+OS_KERNEL="$(uname -s)"
+case "$OS_KERNEL" in
+  Linux)  HOST_OS=linux ;;
+  Darwin) HOST_OS=darwin ;;
+  *)      echo "ERROR: unsupported OS: $OS_KERNEL" >&2; exit 1 ;;
+esac
+
+OS_ARCH="$(uname -m)"
+case "$OS_ARCH" in
+  x86_64 | amd64)   HOST_ARCH=amd64 ;;
+  arm64 | aarch64)  HOST_ARCH=arm64 ;;
+  *)                echo "ERROR: unsupported architecture: $OS_ARCH" >&2; exit 1 ;;
+esac
+
+# A kubectl on PATH only counts as installed if it actually runs on this host
+# (a stale Linux binary on macOS is on PATH but cannot execute).
 kubectl_exists() {
-    command -v kubectl >/dev/null 2>&1
+    command -v kubectl >/dev/null 2>&1 && kubectl version --client >/dev/null 2>&1
 }
 
-# If kubectl is already installed, exit
+# If a working kubectl is already installed, exit
 if kubectl_exists; then
     echo "kubectl is already installed"
     exit 0
@@ -18,8 +37,9 @@ fi
 # Ensure the target directory exists
 mkdir -p "$KUBECTL_DIR"
 
-# Install kubectl
-curl -LO "https://dl.k8s.io/release/$(curl -L -s https://dl.k8s.io/release/stable.txt)/bin/linux/amd64/kubectl"
+# Install kubectl for the detected platform. -f makes curl fail on HTTP errors
+# instead of saving an error page as the binary.
+curl -fLO "https://dl.k8s.io/release/$(curl -fL -s https://dl.k8s.io/release/stable.txt)/bin/${HOST_OS}/${HOST_ARCH}/kubectl"
 chmod +x kubectl
 mv kubectl "$KUBECTL_PATH"