Fix dead run_env in loom_test macro #7

Context: build/rust.bzl:47-51 has the run_env block commented out in loom_test:

# run_env = {
#     "LOOM_LOG": loom_log,
#     "LOOM_LOCATION": str(loom_location),
#     "LOOM_MAX_PREEMPTIONS": str(loom_max_preemptions)
# },

This means the parameters loom_log, loom_max_preemptions are accepted by callers but silently ignored — loom tests run with defaults regardless.

Scope:

1. jj log -p build/rust.bzl (or git log) to find the reason for the commenting.
2. Likely cause: run_env on rust_test requires a different type than a Starlark dict. Try env = { ... } instead, or convert to cmd_args / resolved strings.
3. Re-enable and verify by running kspin loom tests with LOOM_MAX_PREEMPTIONS=5 and confirming wall time increases meaningfully.

Acceptance: loom knobs measurably affect test behavior.