feat: Check for existing bun before downloading

Check for existing bun with same version number, if it exists we skip any futher actions. This avoids downloading cache or bun over the network.

src/action.ts

@@ -7,6 +7,7 @@ import {
   symlinkSync,
   renameSync,
   copyFileSync,
+  existsSync,
 } from "node:fs";
 import { addPath, info, warning } from "@actions/core";
 import { isFeatureAvailable, restoreCache } from "@actions/cache";
@@ -73,6 +74,18 @@ export default async (options: Input): Promise<Output> => {
 
   let revision: string | undefined;
   let cacheHit = false;
+
+  // Check if Bun executable already exists and matches requested version
+  if (!options.customUrl && existsSync(bunPath)) {
+    const existingRevision = await getRevision(bunPath);
+    if (existingRevision && isVersionMatch(existingRevision, options.version)) {
+      revision = existingRevision;
+      cacheHit = true; // Treat as cache hit to avoid unnecessary network requests
+      info(`Using existing Bun installation: ${revision}`);
+    }
+  }
+
+  if (!revision) {
     if (cacheEnabled) {
       const cacheKey = createHash("sha1").update(url).digest("base64");
 
@@ -95,6 +108,7 @@ export default async (options: Input): Promise<Output> => {
       // TODO: remove this, temporary fix for https://github.com/oven-sh/setup-bun/issues/73
       revision = await retry(async () => await downloadBun(url, bunPath), 3);
     }
+  }
 
   if (!revision) {
     throw new Error(
@@ -122,6 +136,27 @@ export default async (options: Input): Promise<Output> => {
   };
 };
 
+function isVersionMatch(
+  existingRevision: string,
+  requestedVersion?: string,
+): boolean {
+  // If no version specified, default is "latest" - don't match existing
+  if (!requestedVersion) {
+    return false;
+  }
+
+  // Non-pinned versions should never match existing installations
+  if (/^(latest|canary|action)$/i.test(requestedVersion)) {
+    return false;
+  }
+
+  const [existingVersion] = existingRevision.split("+");
+
+  const normalizeVersion = (v: string) => v.replace(/^v/i, "");
+
+  return normalizeVersion(existingVersion) === normalizeVersion(requestedVersion);
+}
+
 async function downloadBun(
   url: string,
   bunPath: string,