move command variable substitution to execution ...

This is needed as commands that need to be run need to have generator
settings available before running to avoid inconsistencies with
environment variables passed to the actually ran program and variables
used in substitution.

Fixes #2192

Fixes pre/postGenerateCommands inconsistently using dub environments
(execution was substituted including buildEnvironments, but shell cmd
did not get the build variables for substitution) - now both get
the build variables. Solved by removing code duplication. (env building)

This commit changes the behavior of `dub describe` so that describing
commands settings no longer substitutes variables. Before, known
variables would have been replaced in the output and on unknown
variables `dub describe` would fail with an error
`Target 'x' doesn't exist`

Now exposes some previously private project.d processVars functions