fix: reuse stale tailscale.tgz on self-hosted runners

On persistent self-hosted runners the XDG cache directory survives between
jobs. When GitHub's Actions cache backend doesn't return a hit (no working
backend, evicted entry, etc.), the tailscale.tgz left behind by a previous
job is still on disk, and tc.downloadTool() refuses to overwrite an existing
destination, failing with "Destination file path ... already exists".

Mirror the Windows MSI fix (#259): if the tarball already exists, reuse it
when its checksum matches, otherwise delete and re-download. Only the
cloud-cache-restore path previously avoided this collision, so the Linux
download was unconditional.

Fixes #294

Signed-off-by: TowyTowy <towy@airreps.link>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: TowyTowy

dist/index.js

@@ -472,19 +472,46 @@ async function installTailscaleLinux(
 
   // Download and extract
   const downloadUrl = `${baseUrl}/tailscale_${config.resolvedVersion}_${config.arch}.tgz`;
-  logInfo(config.logMode, `Downloading ${downloadUrl}`);
+  const expectedSha = config.sha256Sum.trim().toLowerCase();
 
   const tarDest = path.join(xdgCacheDir(), "tailscale.tgz");
   fs.mkdirSync(path.dirname(tarDest), { recursive: true });
-  const tarPath = await tc.downloadTool(downloadUrl, tarDest);
 
-  // Verify checksum
-  const actualSha = await calculateFileSha256(tarPath);
-  const expectedSha = config.sha256Sum.trim().toLowerCase();
-  logInfo(config.logMode, `Expected sha256: ${expectedSha}`);
-  logInfo(config.logMode, `Actual sha256: ${actualSha}`);
-  if (actualSha !== expectedSha) {
-    throw new Error("SHA256 checksum mismatch");
+  // Check if the tarball already exists with the correct checksum (for
+  // persistent self-hosted runners). tc.downloadTool refuses to overwrite an
+  // existing destination, so a tarball leaked by a previous job would
+  // otherwise fail with "Destination file path already exists" whenever the
+  // GitHub Actions cache backend doesn't return a hit.
+  let tarPath = tarDest;
+  let needsDownload = true;
+  if (fs.existsSync(tarDest)) {
+    const existingSha = await calculateFileSha256(tarDest);
+    if (existingSha === expectedSha) {
+      logInfo(
+        config.logMode,
+        `Using existing tarball at ${tarDest} (checksum verified)`,
+      );
+      needsDownload = false;
+    } else {
+      logInfo(
+        config.logMode,
+        `Existing tarball checksum mismatch, re-downloading`,
+      );
+      fs.unlinkSync(tarDest);
+    }
+  }
+
+  if (needsDownload) {
+    logInfo(config.logMode, `Downloading ${downloadUrl}`);
+    tarPath = await tc.downloadTool(downloadUrl, tarDest);
+
+    // Verify checksum
+    const actualSha = await calculateFileSha256(tarPath);
+    logInfo(config.logMode, `Expected sha256: ${expectedSha}`);
+    logInfo(config.logMode, `Actual sha256: ${actualSha}`);
+    if (actualSha !== expectedSha) {
+      throw new Error("SHA256 checksum mismatch");
+    }
   }
 
   // Extract to tool path