feat: support packageManager in package.json (#27)

src/action.ts

@@ -1,13 +1,31 @@
 import { tmpdir } from "node:os";
 import * as action from "@actions/core";
 import setup from "./setup.js";
+import { existsSync, readFileSync } from "fs";
+import * as path from "path";
 
 if (!process.env.RUNNER_TEMP) {
   process.env.RUNNER_TEMP = tmpdir();
 }
 
+function readVersionFromPackageJson(): string | undefined {
+  const { GITHUB_WORKSPACE } = process.env;
+  if (!GITHUB_WORKSPACE) {
+    return;
+  }
+  const pathToPackageJson = path.join(GITHUB_WORKSPACE, "package.json");
+  if (!existsSync(pathToPackageJson)) {
+    return;
+  }
+  const { packageManager } = JSON.parse(
+    readFileSync(pathToPackageJson, "utf8")
+  ) as { packageManager?: string };
+  return packageManager?.split("bun@")[1];
+}
+
 setup({
-  version: action.getInput("bun-version") || undefined,
+  version:
+    readVersionFromPackageJson() || action.getInput("bun-version") || undefined,
   customUrl: action.getInput("bun-download-url") || undefined,
 })
   .then(({ version, revision, cacheHit }) => {