cmd/dist: preserve existing GOEXPERIMENTs when running tests with additional experiments

cmd/dist: preserve existing GOEXPERIMENTs when running tests with additional experiments

Some tests require enabling specific Go experiments via the GOEXPERIMENT
, like "jsonv2", "runtimesecret", or "simd".

When running these tests, we should preserve any existing GOEXPERIMENT
settings, so that multiple experiments can be tested together.

I've found this limitation while working in my own Go fork, where in
some situations I pass additional experiments to the tests that alter
the Go runtime and other core packages.

Change-Id: Ib0324cd93282f6993611dea2f0c57d00ab304a33
Reviewed-on: https://go-review.googlesource.com/c/go/+/730360
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>