Bazel `--incompatible_depset_union` breaks latex build

[Zhongming Qu]

This line breaks the Bazel flag --incompatible_depset_union as of Bazel 0.26.0rc16.

Error message was:

ERROR: /home/zhongming/git/LogiOcean/examples/latex/BUILD:9:1: in _latex_pdf rule //examples/latex:helloworld:
Traceback (most recent call last):
        File "/home/zhongming/git/LogiOcean/examples/latex/BUILD", line 9
                _latex_pdf(name = 'helloworld')
        File "/home/zhongming/.cache/bazel/_bazel_zhongming/2aea414f4d422ad78870bb37da8563ec/external/bazel_latex/latex.bzl", line 3, in _latex_pdf_impl
                ctx.actions.run(mnemonic = "LuaLatex", executable ...", <4 more arguments>)
        File "/home/zhongming/.cache/bazel/_bazel_zhongming/2aea414f4d422ad78870bb37da8563ec/external/bazel_latex/latex.bzl", line 16, in ctx.actions.run
                toolchain.kpsewhich.files + toolchain.luatex.files
`+` operator on a depset is forbidden. See https://docs.bazel.build/versions/master/skylark/depsets.html for recommendations. Use --incompatible_depset_union=false to temporarily disable this check.
ERROR: Analysis of target '//examples/latex:helloworld' failed; build aborted: Analysis of target '//examples/latex:helloworld' failed; build aborted