Force -mod=vendor mode

Force -mod=vendor mode

The default should be -mod=readonly for go.mod version 1.13 (present in
go.mod), but that still ends up making a network connection

http://lists.pld-linux.org/mailman/pipermail/pld-devel-en/2021-September/026359.html

go: cloud.google.com/go/storage at v1.12.0: Get "https://proxy.golang.org/cloud.google.com/go/storage/@v/v1.12.0.mod": dial tcp 142.250.203.209:443: connect: connection refused

https://golang.org/ref/mod#build-commands

The -mod flag controls whether go.mod may be automatically updated and
whether the vendor directory is used.

-mod=mod tells the go command to ignore the vendor directory and to
automatically update go.mod, for example, when an imported package is
not provided by any known module.

-mod=readonly tells the go command to ignore the vendor directory and to
report an error if go.mod needs to be updated.

-mod=vendor tells the go command to use the vendor directory. In this
mode, the go command will not use the network or the module cache.

By default, if the go version in go.mod is 1.14 or higher and a vendor
directory is present, the go command acts as if -mod=vendor were used.
Otherwise, the go command acts as if -mod=readonly were used.