fix: cache restore failures (#136)

Fix cache restore failures to to upstream bug.

Fixes #119

src/save.ts

@@ -66,7 +66,10 @@ async function run() {
     }
 
     core.info(`... Saving cache ...`);
-    await cache.saveCache(config.cachePaths, config.cacheKey);
+    // Pass a copy of cachePaths to avoid mutating the original array as reported by:
+    // https://github.com/actions/toolkit/pull/1378
+    // TODO: remove this once the underlying bug is fixed.
+    await cache.saveCache(config.cachePaths.slice(), config.cacheKey);
   } catch (e) {
     core.info(`[warning] ${(e as any).stack}`);
   }