Don't start ssh-agent if it's already running

This allows to run the action multiple times to add multiple keys 
without removing already added ones.

index.js

@@ -21,23 +21,27 @@ try {
     fs.appendFileSync(`${homeSsh}/known_hosts`, '\ngithub.com ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBEmKSENjQEezOmxkZMy7opKgwFB9nkt5YRrYMjNuG5N87uRgg6CLrbo5wAdT/y6v0mKV0U2w0WZ2YB/++Tpockg=\n');
     fs.appendFileSync(`${homeSsh}/known_hosts`, '\ngithub.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOMqqnkVzrm0SdG6UOoqKLsabgH5C9okWi0dh2l9GKJl\n');
     fs.appendFileSync(`${homeSsh}/known_hosts`, '\ngithub.com ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ==\n');
-
-    console.log("Starting ssh-agent");
-
+
     const authSock = core.getInput('ssh-auth-sock');
     const sshAgentArgs = (authSock && authSock.length > 0) ? ['-a', authSock] : [];
-
-    // Extract auth socket path and agent pid and set them as job variables
-    child_process.execFileSync(sshAgent, sshAgentArgs).toString().split("\n").forEach(function(line) {
-        const matches = /^(SSH_AUTH_SOCK|SSH_AGENT_PID)=(.*); export \1/.exec(line);
-
-        if (matches && matches.length > 0) {
-            // This will also set process.env accordingly, so changes take effect for this script
-            core.exportVariable(matches[1], matches[2])
-            console.log(`${matches[1]}=${matches[2]}`);
-        }
-    });
-
+
+    if (child_process.spawnSync(sshAdd, ['-l'], { env: { ...process.env, SSH_AUTH_SOCK: authSock || env['SSH_AUTH_SOCK'] } }) !== 0) {
+        console.log('ssh-agent is already running, not starting a new one')
+    } else {
+      console.log("Starting ssh-agent");
+
+       // Extract auth socket path and agent pid and set them as job variables
+      child_process.execFileSync(sshAgent, sshAgentArgs).toString().split("\n").forEach(function(line) {
+          const matches = /^(SSH_AUTH_SOCK|SSH_AGENT_PID)=(.*); export \1/.exec(line);
+
+          if (matches && matches.length > 0) {
+              // This will also set process.env accordingly, so changes take effect for this script
+              core.exportVariable(matches[1], matches[2])
+              console.log(`${matches[1]}=${matches[2]}`);
+          }
+      });
+    }
+
     console.log("Adding private key(s) to agent");
 
     privateKey.split(/(?=-----BEGIN)/).forEach(function(key) {