js.bzl: Stop using invalid join call
js.bzl: Stop using invalid join call

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: I639948584a65decfbe873c214c027290d9401c56

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