fix(webhook-github-app): Allow new lines in base64 key for module webhook-github-app (#3714)

We store the `github_app.key_base64` configuration settings with
newlines (nothing unusual, actually). This breaks the
`webhook-github-api` module, which then reports the following when
running `terraform apply`:

```
module.webhook-github-app.null_resource.update_app (local-exec): Executing: ["bash" "-c" ".terraform/modules/webhook-github-app/modules/webhook-github-app/bin/update-app.sh -e https://REDACTED/webhook -s REDACTED -a REDACTED -k FIRSTLINE\nSECONDLINE\nAND_SO_ON==\n"]
module.webhook-github-app.null_resource.update_app (local-exec): Could not read private key from /dev/fd/63
module.webhook-github-app.null_resource.update_app (local-exec): 40975344027F0000:error:1608010C:STORE routines:ossl_store_handle_load_result:unsupported:crypto/store/store_result.c:151:
module.webhook-github-app.null_resource.update_app (local-exec): {"message":"A JSON web token could not be decoded","documentation_url":"https://docs.github.com/rest"}gh: A JSON web token could not be decoded (HTTP 401)
```

The root cause is that `base64 -d` cannot decode the argument passed
through `-k`. Fix that by telling `echo` to leave the newlines in place,
so that `base64 -d` can deal with them. Adding `tr -d` is not necessary
in this case.

Co-authored-by: Niek Palm <npalm@users.noreply.github.com>

Author: imphil

modules/webhook-github-app/bin/update-app.sh

@@ -76,7 +76,7 @@ if [ -z "$APP_PRIVATE_KEY_BASE64" ]; then
   APP_PRIVATE_KEY_BASE64=$(cat $APP_PRIVATE_KEY_FILE | base64 | tr -d '\n')
 fi
 
-SIGNATURE=$(echo -n "$HEADER.$PAYLOAD" | openssl dgst -sha256 -sign <(echo "$APP_PRIVATE_KEY_BASE64" | base64 -d) | base64 | tr -d '\n')
+SIGNATURE=$(echo -n "$HEADER.$PAYLOAD" | openssl dgst -sha256 -sign <(echo -e "$APP_PRIVATE_KEY_BASE64" | base64 -d) | base64 | tr -d '\n')
 
 JWT_TOKEN="$HEADER.$PAYLOAD.$SIGNATURE"