Avoid pulling plugins from the Terraform online repositry

We'd like to test what we ship, so we should not allow terraform
to pull from the online repository additional plugins, as that would
hide issues in the state that we're trying to test in the CI.

ci/infra/testrunner/platforms/terraform.py

@@ -52,6 +52,8 @@ def _provision_platform(self, masters=-1, workers=-1):
         if self.conf.terraform.plugin_dir:
             logger.info(f"Installing plugins from {self.conf.terraform.plugin_dir}")
             init_cmd += f" -plugin-dir={self.conf.terraform.plugin_dir}"
+        else:
+            init_cmd += f" -get-plugins=false"
         self._run_terraform_command(init_cmd)
 
         self._run_terraform_command("version")