Support go1.13 -trimpath option

[aprice]

Quick change to allow passing through the new -trimpath build option added in Go 1.13.

[dropwhile]

FYI - This worked great for me. Thanks @aprice.

[mvdan]

Here's a temporary workaround:

export GOFLAGS="-trimpath"

gox -whatever -flags

Arguably, we could use GOFLAGS for everything, not needing to expose all flags in gox itself again.