The existing implementation can call to T.Setenv in T.Run even after
calling to T.Parallel, so I changed it to cause a panic in that case.

Fixes #55128

Change-Id: Ib89d998ff56f00f96a5ca218af071bd35fdae53a
Reviewed-on: https://go-review.googlesource.com/c/go/+/431101


Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>