cmd/compile: fix test to use correct go binary
cmd/compile: fix test to use correct go binary

Use internal/testenv package to get the right go binary.
Otherwise, I think we're just grabbing an old one from the environment.

Fixes #22560.

Change-Id: Id5b743b24717e15ec8ffbcfae4dc3e5f6a87b9a9
Reviewed-on: https://go-review.googlesource.com/76090
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

nawawi/go,akutz/go,codestation/go,golang/go,golang/go,josharian/go,codestation/go,CAFxX/go,akutz/go,nawawi/go,Cofyc/go,nawawi/go,cloudflare/go,CAFxX/go,codestation/go,codestation/go,josharian/go,josharian/go,josharian/go,Cofyc/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,christopher-henderson/Go,cloudflare/go,Cofyc/go,CAFxX/go,codestation/go,golang/go,codestation/go,Cofyc/go,CAFxX/go,golang/go,Cofyc/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,akutz/go,codestation/go,nawawi/go,akutz/go,nawawi/go,CAFxX/go,Cofyc/go,akutz/go,josharian/go,Cofyc/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,josharian/go,Cofyc/go,golang/go,codestation/go,josharian/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go