Fix rustdoc path in rustdoc_test with --nolegacy_external_runfiles option

[oxidase]

Using File.path breaks builds with --nolegacy_external_runfiles option
as the gnerated script has use external/rust/bin/rustdoc that
referrers to legacy runfiles and File.short_path uses ../rust/bin/rustdoc

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[oxidase]

@googlebot I signed it!

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[damienmg]

LGTM

[damienmg]

Thanks!