Fix output malformed when wrapper enabled

Presently using a command such as `terraform output -json | jq` does not
work with the wrapper enabled, as it is by default.

In order to consume terraform's output having set it up with this
Action, it is necessary either to disable the wrapper (`with:
terraform_wrapper: false`) or run it in its own Actions step with an
explicit `id` (e.g. `id: foo`) so that it can be referred to and consumed
(`${{steps.foo.outputs.stdout}}` et al.) in later steps.

This seems to be the result of much confusion (issues passim) and is not
at all easy (#338) to debug/diagnose and come to the realisation that
it's due to the wrapper, or even that such a thing exists.

@austinvalle identified the issue as being due to the `@actions/exec`
package writing the spawned command to stdout (along with then its
actual stdout). This has previously been reported upstream in
actions/toolkit#649; I've proposed actions/toolkit#1573 to fix it.

This commit aims to address the issue for `setup-terraform` in the
meantime by silencing `@actions/exec` and then writing out to stdout &
stderr from the listener buffers, which it writes to without this
additional logging.

Closes #20, #80, #85, #149, #338, and probably more.

wrapper/terraform.js

@@ -33,9 +33,15 @@ async function checkTerraform () {
   const args = process.argv.slice(2);
   const options = {
     listeners,
-    ignoreReturnCode: true
+    ignoreReturnCode: true,
+    silent: true, // work around github.com/actions/toolkit#649
   };
   const exitCode = await exec(pathToCLI, args, options);
+
+  // Pass-through stdout/err as `exec` won't due to `silent: true` option
+  process.stdout.write(stdout.contents);
+  process.stderr.write(stderr.contents);
+
   core.debug(`Terraform exited with code ${exitCode}.`);
   core.debug(`stdout: ${stdout.contents}`);
   core.debug(`stderr: ${stderr.contents}`);