cmd/go: ensure that runtime/cgo is not stale in TestExecBuildX
cmd/go: ensure that runtime/cgo is not stale in TestExecBuildX

Fixes #29004

Change-Id: Ifc8330b47fbec6f902057cb9b6db44f2c0082056
Reviewed-on: https://go-review.googlesource.com/c/go/+/183838
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

nawawi/go,golang/go,CAFxX/go,akutz/go,akutz/go,akutz/go,CAFxX/go,golang/go,golang/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,akutz/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,akutz/go,akutz/go