workflows: Use setup-terraform install latest

[darkowlzz]

Terraform v1.10.0 had a bug in apply when reading variables from environment, refer hashicorp/terraform#36106 for details. This resulted in all of our CI setup that uses terraform to fail with the following error:

Error: Can't change variable when applying a saved plan

The variable tags cannot be set using the -var and -var-file options when
applying a saved plan file, because a saved plan includes the variable values
that were set when it was created. The saved plan specifies
"{\"environment\"=\"github\", \"ci\"=\"true\", \"repo\"=\"pkg\",
\"createdat\"=\"x2024-12-10_12h09m54s\"}" as the value whereas during apply
the value object with 4 attributes was set by an environment variable. To
declare an ephemeral variable which is not saved in the plan file, use
ephemeral = true.

Terraform v1.10.1 fixes this. But in all of our CI, we use ubuntu-latest (22.04), which was updated to terraform v1.10.0 as of last week, refer https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2204-Readme.md.

Use setup-terraform to install the latest version of terraform.
Unlike Go, which is used on a daily basic by all the Flux developers, the terraform code and infrastructure gets maintenance updates occasionally. Hence, keeping it on a rolling version, which would make any failure due to incompatibility more visible for now.

CI results:

• GCP https://github.com/fluxcd/pkg/actions/runs/12258563799
• AWS https://github.com/fluxcd/pkg/actions/runs/12258684393/job/34199032690

[stefanprodan]

Thanks @darkowlzz