Use gtar(1) not tar(1) due to dynamic compression support

Index: src/libstore/download.cc
--- src/libstore/download.cc.orig
+++ src/libstore/download.cc
@@ -918,7 +918,7 @@ CachedDownloadResult Downloader::downloadCached(
             Path tmpDir = createTempDir();
             AutoDelete autoDelete(tmpDir, true);
             // FIXME: this requires GNU tar for decompression.
-            runProgram("tar", true, {"xf", store->toRealPath(storePath), "-C", tmpDir, "--strip-components", "1"});
+            runProgram("gtar", true, {"xf", store->toRealPath(storePath), "-C", tmpDir, "--strip-components", "1"});
             unpackedStorePath = store->addToStore(name, tmpDir, true, htSHA256, defaultPathFilter, NoRepair);
         }
         replaceSymlink(unpackedStorePath, unpackedLink);