Move cache save to runs.post and exit early (#60)

src/cache-save.ts

@@ -0,0 +1,15 @@
+import { saveCache } from "@actions/cache";
+import { getState, warning } from "@actions/core";
+import { CacheState } from "./action";
+
+(async () => {
+  const state: CacheState = JSON.parse(getState("cache"));
+  if (state.cacheEnabled && !state.cacheHit) {
+    try {
+      await saveCache([state.bunPath], state.url);
+      process.exit(0);
+    } catch (error) {
+      warning("Failed to save Bun to cache.");
+    }
+  }
+})();