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

Index: src/nix-prefetch-url/nix-prefetch-url.cc
--- src/nix-prefetch-url/nix-prefetch-url.cc.orig
+++ src/nix-prefetch-url/nix-prefetch-url.cc
@@ -193,7 +193,7 @@ static int _main(int argc, char * * argv)
                     runProgram("unzip", true, {"-qq", tmpFile, "-d", unpacked});
                 else
                     // FIXME: this requires GNU tar for decompression.
-                    runProgram("tar", true, {"xf", tmpFile, "-C", unpacked});
+                    runProgram("gtar", true, {"xf", tmpFile, "-C", unpacked});
 
                 /* If the archive unpacks to a single file/directory, then use
                    that as the top-level. */