Randomize SSH auth socket, kill agent to support non-ephemeral, self hosted runners (@thommyhh, #27)

Thanks to @thommyhh for this contribution!

Unless the `SSH_AUTH_SOCK` is configured explicitly, this change will make the SSH agent use a random file name for the socket. That way, multiple, concurrent SSH agents can be used on non-ephemeral, self-hosted runners.

A new post-action step will automatically clean up the running agent at the end of a job.

Be aware of the possible security implications: Two jobs running on the same runner might be able to access each other's socket and thus access repositories and/or hosts.

cleanup.js

@@ -0,0 +1,10 @@
+const core = require('@actions/core')
+const { execSync } = require('child_process')
+
+try {
+    // Kill the started SSH agent
+    console.log('Stopping SSH agent')
+    execSync('kill ${SSH_AGENT_PID}', { stdio: 'inherit' })
+} catch (error) {
+    core.setFailed(error.message)
+}