cmd/buildlet: ignore symlinks for now

As part of getting the x/build tests passing (trybots + post-submit),
ignore the symlinks that are in the x/build repo for now.

We can add support when we actually need them for something. But I
imagine any test that needs them can & does just create them as
needed.

Updates golang/go#14594

Change-Id: I6a0584d35c8d0fc91b3cdc8114b473df7f0268c3
Reviewed-on: https://go-review.googlesource.com/c/157439
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/buildlet/buildlet.go

@@ -810,6 +810,11 @@ func untar(r io.Reader, dir string) (err error) {
 				return err
 			}
 			madeDir[abs] = true
+		case mode&os.ModeSymlink != 0:
+			// TODO: ignore these for now. They were breaking x/build tests.
+			// Implement these if/when we ever have a test that needs them.
+			// But maybe we'd have to skip creating them on Windows for some builders
+			// without permissions.
 		default:
 			return badRequest(fmt.Sprintf("tar file entry %s contained unsupported file type %v", f.Name, mode))
 		}