fix(integration): make integration test pass on macOS

[xytan0056]

1. fixes CI
   https://github.com/uber/tango/actions/runs/29140659260/job/86512997744#step:3:520
   GetChangedTargets computes two graphs in parallel; both raced to download the same bazel release, corrupting it (exit status 22) then hanging to the deadline.

2. And mac

    --- FAIL: TestIntegration_GetChangedTargets/changed_only (1.85s)
        integration_test.go:285: 
                Error Trace:    integration/integration_test.go:285
                                  integration/integration_test.go:296
                                  integration/integration_test.go:420
                Error:          Received unexpected error:
                                code:unknown message:failed to get target graph #1: create bazel client: detect bazel executable: cache dir: $HOME is not defined

the inner bazel failed with $HOME is not defined and SSL cert errors while re-downloading.

Fix:

• Pass HOME through to the sandbox so the inner bazel reuses the release the outer bazel test already downloaded — no in-test download, no race.
• Set bazel_command to the repo's pinned tools/bazel so inner and outer share one cache instead of fetching bazelisk from GitHub latest. ${BAZEL} is alreayd called in makefile rule, so by the time integ test is run, it's already downloaded
• Drop the per-server XDG_CACHE_HOME override.

##test
verified make test-integration passes locally on mac
verified CI