Painted source code isn't updated in coverage jobs

[cottsay]

The painted source code view on the coverage jobs isn't being updated with fresh coverage data. The regression appears to have been caused by the switch from gcov to lcov in #481.

The nightly_linux_coverage job has painted code from the day that PR was merged, and the nightly_linux_foxy_coverage job which was added after the referenced PR was merged, has no painted source code available at all.

This comment on a ticket for the Cobertura plugin for a similar issue seems to indicate that this behavior can manifest when the <sources> section of the coverage XML isn't correctly populated. The current builds populate it simply with <source>.</source>. Looking at the diff in #481, it appears that some post-processing was done on those entries which leads me to believe that there was much more data there prior to the the switch to lcov.