scripts: namespace dev_cli tmp dir per user and drop dead BUILD_DIR

The shared /tmp/cloud-hypervisor/ path is created by the first user to
run dev_cli.sh and owned by them, so other users on the same host fail
the +x+w check and cannot chmod it back. Move both tmp paths under
/tmp/cloud-hypervisor-${USER}/ so each user gets their own tree.

While here, fold the local BUILD_DIR in build_container() into the
existing (previously unused) CLH_CTR_BUILD_DIR, which ensure_build_dir()
already creates.

Assisted-by: Claude:Opus-4.7
Signed-off-by: Bo Chen <bchen@crusoe.ai>

scripts/dev_cli.sh

@@ -20,7 +20,7 @@ CLH_ROOT_DIR=$(cd "${CLH_SCRIPTS_DIR}/.." && pwd)
 CLH_BUILD_DIR="${CLH_ROOT_DIR}/build"
 CLH_CARGO_TARGET="${CLH_BUILD_DIR}/cargo_target"
 CLH_DOCKERFILE="${CLH_SCRIPTS_DIR}/../resources/Dockerfile"
-CLH_CTR_BUILD_DIR="/tmp/cloud-hypervisor/ctr-build"
+CLH_CTR_BUILD_DIR="/tmp/cloud-hypervisor-${USER}/ctr-build"
 CLH_INTEGRATION_WORKLOADS="${HOME}/workloads"
 
 # Container paths
@@ -649,10 +649,7 @@ cmd_tests() {
 build_container() {
     ensure_build_dir
 
-    BUILD_DIR=/tmp/cloud-hypervisor/container/
-
-    mkdir -p $BUILD_DIR
-    cp "$CLH_DOCKERFILE" $BUILD_DIR
+    cp "$CLH_DOCKERFILE" "$CLH_CTR_BUILD_DIR"
 
     [ "$(uname -m)" = "aarch64" ] && TARGETARCH="arm64"
     [ "$(uname -m)" = "x86_64" ] && TARGETARCH="amd64"
@@ -660,9 +657,9 @@ build_container() {
     $DOCKER_RUNTIME build \
         --target dev \
         -t "$CTR_IMAGE" \
-        -f $BUILD_DIR/Dockerfile \
+        -f "$CLH_CTR_BUILD_DIR/Dockerfile" \
         --build-arg TARGETARCH="$TARGETARCH" \
-        $BUILD_DIR
+        "$CLH_CTR_BUILD_DIR"
 }
 
 cmd_build-container() {