Override dub dflags for a single source file only

[Per Nordlöw]

In dub, is it possible to override the dflags, in my case `-cov`, 
for a single file only without it affecting the rest of the 
compiling and linking? Currently, dub enforces static linking of 
the dependencies when the `-cov` flag  is set via `dflags "-cov"` 
in dub.sdl which makes our build fail.