ci: pin terraform until planning bug is resolved

[ryancragun]

Terraform 1.4.x introduced an issue that prevents some enos resources from planning correctly. This pins the version of Terraform until the issue between provider and Terraform 1.4.x is resolved.

╷
│ Error: exit status 1
│
│ Error: Provider produced invalid plan
│
│ Provider "app.terraform.io/hashicorp-qti/enos" planned an invalid value for
│ module.get_vault_cluster_ips.enos_remote_exec.get_leader_private_ip.environment:
│ sensitive planned value does not match config value.
│
│ This is a bug in the provider, which should be reported in the provider's own
│ issue tracker.
│ 
│
│   (source code not available)