[gcp, aws, digitalocean] Ensure the env var `MEILI_SERVER_PROVIDER` is being sent

[brunoocasali]

We need to ensure that we are setting our environment variable correctly.

Like we used to do in the cloud-scripts: https://github.com/meilisearch/cloud-scripts/blob/56a7c2630c1a508e5ad0c0ba1d8cfeb8d2fa9ae0/scripts/providers/gcp/cloud-config.yaml#L33

[alallema]

Yes, the variable is always set here then substituted here.
The change is that now the names are no longer the same because we use the source denomination: amazon-ebs, digitalocean, googlecompute.

[brunoocasali]

Thanks @alallema 🎉