ci: pin code format terraform version

The formatter in Terraform 0.14 is currently not stable and keeps
formatting `local.tf` until the syntax breaks.

Use the same version of Terraform as the rest of the code.

Author: zimbatm

.github/workflows/lint.yml

@@ -14,7 +14,7 @@ jobs:
     name: Code Format
     runs-on: ubuntu-latest
     container:
-      image: hashicorp/terraform:latest
+      image: hashicorp/terraform:0.12.29
     steps:
       - uses: actions/checkout@master
       - run: terraform fmt --recursive -check=true