build: Enable -mod build flag to be toggled via environment variable
build: Enable -mod build flag to be toggled via environment variable

This change will enable builds to be performed with the option of
passing the -mod flag to 'go build'. This can be done by optionally
setting the MODMODE environment variable prior to building.

For example, toggling the -mod setting for the build can be
accomplished as follows:

```
$ MODMODE=vendor ./build
```

Signed-off-by: Ryan Tidwell <6e96aaaf0a9256611f00853daaeaaab7ebc073df@suse.com>

intel/multus-cni,Intel-Corp/multus-cni,intel/multus-cni,Intel-Corp/multus-cni