fix(toolchain/go): configurable go subcommands
fix(toolchain/go): configurable go subcommands

So far, only the main go binary was configurable with
`m.in/toolchain/go/program/go`. You can now also configure
sub-commands (go test, go build, ...) and use a different
tool for a single sub-commands.

Example:
```
m.in/toolchain/go/bin/go-test = $(m.in/cwd)bin/ginkgo
```

farjump/Makefile.in,farjump/Makefile.in,farjump/Makefile.in