[BUG] Adds Github Actions PR permission so that PR comments work

[SrzStephen]

When you try to follow the instructions in https://developer.hashicorp.com/terraform/tutorials/aws/preview-environments-vercel#create-preview-environment you get an error in the Create comment with Terraform output action, which fails with RequestError [HttpError]: Resource not accessible by integration seen here.

This is fixable by adding the pull-requests: write permission to github actions as shown here

Note: using the issues permission won't work as it's not scoped correctly as shown here

[hashicorp-cla]

All committers have signed the CLA.