feat: ts_proto_library: improve rules_js compatibility

[tsawada]

rules_js's js_binary targets need BAZEL_BINDIR env variable when running as bazel's actions.
With this change, ts_proto_library can be used with rules_js, if users provide rules_js version of protoc-gen-ts under //protoc-gen-ts/bin like this:

load("@npm//:protoc-gen-ts/package_json.bzl", "bin")

bin.protoc_gen_ts_binary(
    name = "protoc-gen-ts",
    visibility = ["//visibility:public"],
)

This PR does not migrate anything to rules_js. However, this small change makes protoc-gen-ts practically usable with rules_js, while being totally harmless to rules_nodejs users.
ref #176