misc/cgo/testshared: pass -x flag only to commands supporting it

Running testshared with the -testx flag leads to:

./testshared.test -testx -testwork
+ mkdir -p /tmp/shared_test125221103
shared_test.go:79: executing go env -x GOROOT failed exit status 2:
flag provided but not defined: -x
usage: go env [-json] [-u] [-w] [var ...]
Run 'go help env' for details.
panic: executing go env -x GOROOT failed exit status 2:
flag provided but not defined: -x
usage: go env [-json] [-u] [-w] [var ...]
Run 'go help env' for details.

Change-Id: Id299979487c021e9ad1d57f5f7088d935830a6a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/371614


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Cherry Mui <cherryyz@google.com>