test(firecracker): pin INFRA_BUILD off in the default-pull test

The coverage CI job exports BOT_BOTTLE_INFRA_BUILD=local so the integration
suite builds infra/agent images from source. But that job also runs the unit
suite, and the ambient =local leaked into test_default_pulls_artifact_without_
docker — it drove ensure_built() down the local Docker-build branch, so the
"build_image not called / artifact pulled" assertion failed (build_image
called 3 times).

Pin the var off in that test so it exercises the default pull path regardless
of the runner's ambient env (mirrors test_local_mode_builds_deps_before_infra,
which pins it on). Full unit suite verified green under BOT_BOTTLE_INFRA_BUILD
=local.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A9qa3xoavjQScufDfZaXKR

tests/unit/test_firecracker_infra_vm.py

@@ -100,7 +100,11 @@ class TestRegistryVolume(unittest.TestCase):
 class TestEnsureBuilt(unittest.TestCase):
     def test_default_pulls_artifact_without_docker(self):
         # PRD 0069 Stage 2: the launch host pulls the prebuilt rootfs; no Docker.
-        with patch.object(infra_vm.docker_mod, "build_image") as build, \
+        # Pin BOT_BOTTLE_INFRA_BUILD off: the coverage CI job exports it =local
+        # for the integration suite, and that ambient value would otherwise send
+        # this default-path test down the local Docker-build branch.
+        with patch.dict(os.environ, {"BOT_BOTTLE_INFRA_BUILD": ""}), \
+                patch.object(infra_vm.docker_mod, "build_image") as build, \
                 patch.object(infra_vm.infra_artifact, "ensure_artifact_gz") as pull:
             infra_vm.ensure_built()
         build.assert_not_called()