Build failed on Travis-CI: CS0016 could not write to output file

The issue happens during building .NET Standard class library project on Dockerized Ubuntu where SourceLink tries to create and write to **(Project Name)/obj/(Release or Debug)/netstandard2.0/(Project Name).sourcelink.json** file. The error message is:

> CSC : error CS0016: Could not write to output file '**(Project Name)/obj/(Release or Debug)/netstandard2.0/(Project Name).sourcelink.json** file' -- 'Could not find file '(Project Name)/obj/(Release or Debug)/netstandard2.0/(Project Name).sourcelink.json'.'

See [full build log](https://travis-ci.org/rvhuang/linq-to-astar/builds/403420427) for detail. Full project file can be found [here](https://github.com/rvhuang/linq-to-astar/blob/wip-sourcelink/src/Heuristic.Linq/Heuristic.Linq.csproj).

To date, the issue is not observed on other platforms such as Windows and Mac OS.