Fix deploy triggered by workflow_dispatch

This would probably be equivalent, but I've never written a GitHub Action so I took the safe route here.

`const commitId = isPullRequest ? payload.pull_request.head.sha : github.sha;`

Fixes #7

Author: mkarnicki

index.js

@@ -10,7 +10,7 @@
     const fullRepositoryName = payload.repository.full_name; // like "Codertocat/Hello-World"
 
     const isPullRequest = payload.pull_request !== undefined;
-    const commitId = isPullRequest ? payload.pull_request.head.sha : payload.head_commit.id; // like "ec26c3e57ca3a959ca5aad62de7213c562f8c821"
+    const commitId = isPullRequest ? payload.pull_request.head.sha : (payload.head_commit ? payload.head_commit.id : github.sha); // like "ec26c3e57ca3a959ca5aad62de7213c562f8c821"
     const branchName = isPullRequest ? payload.pull_request.head.ref : payload.ref.replace(/^refs\/heads\//, ''); // like "my/branch_name"
     console.log(`🎋 On branch '${branchName}', head commit ${commitId}`);