Error downloading [http://zlib.net/zlib-1.2.11.tar.gz]

Hi,

it seems the current `zlib` dependency link is dead and thus the
```
$ bazel build @dbx_build_tools//build_tools:bzl
```
command, as references in the `README.md` fails with ↓

```
ERROR:
/home/user/.cache/bazel/_bazel_user/d5f246ac2500b7dd646c66395fcf22ef/external/org_python_cpython_39/BUILD.bazel:242:11:
@org_python_cpython_39//:builtin_extensions depends on @net_zlib//:zlib in
repository @net_zlib which failed to fetch. no such package '@net_zlib//':
java.io.IOException: Error downloading [http://zlib.net/zlib-1.2.11.tar.gz] to
/home/user/.cache/bazel/_bazel_user/d5f246ac2500b7dd646c66395fcf22ef/external/net_zlib/temp3678545797472261272/zlib-1.2.11.tar.gz:
GET returned 404 Not Found
```

AFAIS, it's set here ↓
https://github.com/dropbox/dbx_build_tools/blob/16db447a80545d1ae6236fa436878423e5d2ad9a/build_tools/bazel/external_workspace.bzl#L20

it seems the zlib project moved it to https://zlib.net/fossils/zlib-1.2.11.tar.gz

Cheers