js.bzl: Stop using incompatible join method
js.bzl: Stop using incompatible join method

Bazel 0.27 flipped incompatible Starlark flag:
--incompatible_string_join_requires_strings, see: [1] for
more details.

[1] https://github.com/bazelbuild/bazel/issues/7802

Change-Id: Id8588f65ba34a084d82f282da180fb8b35ef48ac

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