fix: switch tools from host to exec cfg

[peakschris]

Bazel 7 with --incompatible_disable_starlark_host_transition (which we use by default) is giving the error:

ERROR: Traceback (most recent call last):
        File "D:/udu/b/qpawu7hm/external/rules_antlr/antlr/antlr2.bzl", line 71, column 28, in <toplevel>
                "_tool": attr.label(
Error in label: 'cfg = "host"' is deprecated and should no longer be used. Please use 'cfg = "exec"' instead.

I'm not sure your plan for accepting PRs into your hoist branch, but if suitable, I would like to contribute as we find issues in our adoption of rules_antlr

[albertocavalcante]

LGTM! Thanks @peakschris! Appreciate your contribution.

Context: bazelbuild/bazel#17032