Variable substitution in launch.json for build directory path

[nsubtil]

As a follow-on to the discussion in #559, it would be useful to enable variable substitution for ${cmake.buildDirectory} in launch.json.

The intended use case would be applications that require cwd to be somewhere inside the build directory, or that require a path within the build directory to be passed in as a command-line argument (e.g., for loading data generated during the build).

[HorstBaerbel]

This is very much needed for e.g. cwd and would be useful in tasks.json too.

[andreeis]

Closing as dupe of #1165. Reactivate with more details if the solutions presented there don't work for you.