[bbagent] Avoid mutating exe.cmd[0]
[bbagent] Avoid mutating exe.cmd[0]

Not the bug we're looking for, but a bug nonetheless.

R=tandrii@chromium.org,50b2a565e2e78c292794832469a30ce5abc9959c@google.com

Bug: 1015181
Change-Id: I1298077e3a2138486c6bd0c52df928815cfa350f
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2207821
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go