[BUG] overwrites the authorized_keys file in self-hosted gh runner

[yatharthranjan]

Hi,

We are using a self-hosted GH runner and seems like we lost SSH access to our instance because this action overwrites the authorized_keys files when limit-access-to-actor set to true. I think it is because of this line

action-tmate/src/index.js

Line 147 in a283f94

await fs.promises.writeFile(authorizedKeysPath, keys.data.map(e => e.key).join('\n'))

This might be fine on GH hosted runners as they are ephermeral, but for self-hosted runner we have a single instance which is re-used across multiple runs. For this we need SSH access to configure and administer the instance and overwriting SSH keys is not useful. For now we have changed to a new user specific for running the GH runner application so it does not interfere with our primary user account.

I think this should be updated to append to the authorized_keys file instead of replacing it. So any existing keys will also work. Thanks.