go.mk: remove submodule and initialize through make

[sauterp]

Testing

❯ make build
Cloning into 'go.mk'...
remote: Enumerating objects: 311, done.
remote: Counting objects: 100% (197/197), done.
remote: Compressing objects: 100% (95/95), done.
remote: Total 311 (delta 132), reused 138 (delta 93), pack-reused 114
Receiving objects: 100% (311/311), 62.32 KiB | 3.89 MiB/s, done.
Resolving deltas: 100% (175/175), done.
mkdir -p '/home/sauterp/src/github.com/exoscale/vault-plugin-secrets-exoscale/bin'
'/home/sauterp/bin/go' build \
   \
  -ldflags "-s -w -X github.com/exoscale/vault-plugin-secrets-exoscale/version.Version=dev -X github.com/exoscale/vault-plugin-secrets-exoscale/version.Commit=0ee9199" \
   \
  -o '/home/sauterp/src/github.com/exoscale/vault-plugin-secrets-exoscale/bin/' \
  './cmd/vault-plugin-secrets-exoscale'

[shortcut-integration]

This pull request has been linked to Shortcut Story #88913: go.mk: provide alternative to submodule approach.