cmd/testwrapper: make test tolerant of a GOEXPERIMENT being set

Otherwise it generates an syntactically invalid go.mod file
and subsequently fails.

Updates #18884

Change-Id: I1a0ea17a57b2a37bde3770187e1a6e2d8aa55bfe
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f