fix(harness): unique chain id for the major-upgrade suite too

[bdchatham]

#446 wired runChainID into the load/release/chaos suites but missed TestChainUpgrade — it provisions a 4-validator chain on a static SEI_CHAIN_ID (upg), so it's exposed to the same stale-genesis halt on any same-id collision (back-to-back run, or a SIGKILL leaving genesis behind). It passed earlier only on a clean slate. Its own doc already calls SEI_CHAIN_ID "(base)"; this makes the code match.

Completes the per-run unique chain id across all four suites (load/release/chaos already done in #446).

test/integration/upgrade_test.go:96  chainID := runChainID(mustEnv(t, "SEI_CHAIN_ID"))

This is the identical one-line pattern #446's xreview ratified unanimously (sei-network dissenter held, idiomatic RATIFY). go vet -tags integration + gofmt clean. PLT-762 / PLT-758.

[cursor]

PR Summary

Low Risk
Single-line integration harness change with no production auth, data, or runtime logic touched.

Overview
TestChainUpgrade now derives the provisioned chain id via runChainID on the SEI_CHAIN_ID base env var, aligning with load/release/chaos suites from #446.

This avoids stale-genesis / same-id collisions when the upgrade harness runs back-to-back or leaves persisted genesis behind after an abrupt stop.

^{Reviewed by Cursor Bugbot for commit 01056d1. Bugbot is set up for automated code reviews on this repo. Configure here.}