abuild.conf: add -modcacherw to default Go build flags

By default, the Go module cache is read-only. This become a problem when cleaning up the build directory from within abuild. For this reason, many existing aports for Go software employ one of the following solutions: (1) passing -modcacherw manually (2) setting the chmod-clean option or (3) overwriting default_cleanup_srcdir. By solving this problem globally we make it easier to create packages for Go software and ensure consistency in regards to cleaning the module cache.