The merge_group coverage job did not set RUST_MIN_STACK, so test threads
ran with the default 2MiB stack while the regular test job (and the
production runtime in debug) use 8MiB. The deeper PromQL planning chain
from #9494 overflows 2MiB under llvm-cov instrumentation, aborting
tests::promql_test::anon_promql_ratio_repro::case_1_test_with_standalone
with SIGABRT (12/12 failures across three merge-queue candidates).
Align the coverage job with the existing 8MiB test policy.
Signed-off-by: discord9 <discord9@163.com>