downloadTarball should not redownload the source file if not needed

If 1) the store path for the unpacked source is valid, and 2) the file
at the source has not changed, downloadTarball shouldn't redownload the
source file.

src/libfetchers/tarball.cc

@@ -146,8 +146,7 @@ DownloadTarballResult downloadTarball(
             .immutableUrl = maybeGetStrAttr(cached->infoAttrs, "immutableUrl"),
         };
 
-    auto res = downloadFile(store, url, name, locked, headers);
-    assert(res.storePathValid);
+    auto res = downloadFile(store, url, name, locked, headers, cached.has_value());
 
     std::optional<StorePath> unpackedStorePath;
     time_t lastModified;
@@ -158,6 +157,11 @@ DownloadTarballResult downloadTarball(
     } else {
         Path tmpDir = createTempDir();
         AutoDelete autoDelete(tmpDir, true);
+        if (!res.storePathValid) {
+            debug("source etag didn't match unpacked etag, or server returned 304 with a different etag.");
+            res = downloadFile(store, url, name, locked);
+            assert(res.storePathValid);
+        }
         unpackTarfile(store->toRealPath(res.storePath), tmpDir);
         auto members = readDirectory(tmpDir);
         if (members.size() != 1)