feat: add bun- prefix to cache keys (#160)

* feat: add bun- prefix to cache keys

Extract cache key generation into a shared `getCacheKey` function in
utils.ts that both action.ts and cache-save.ts import.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* [autofix.ci] apply automated fixes

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/utils.ts

@@ -1,8 +1,13 @@
 import { debug, warning } from "@actions/core";
 import { info } from "node:console";
+import { createHash } from "node:crypto";
 import { existsSync, readFileSync, renameSync } from "node:fs";
 import { resolve, basename } from "node:path";
 
+export function getCacheKey(url: string): string {
+  return `bun-${createHash("sha1").update(url).digest("base64")}`;
+}
+
 export async function request(
   url: string,
   init?: RequestInit,