The `$DUB_BUILD_PATH` variable was added

The `$DUB_BUILD_PATH` variable is now defined inside the `postBuildCommands`
section. It contains the absolute path in which the package was built, and can
be used to copy by-products of the build process to their intended locations.

For example, if an executable exports symbols, you will want to make the
resulting import library and symbols export file available somewhere. That can
be done with a `dub.json` section like this:
-------
    "postBuildCommands-windows": [
        "copy /y $DUB_BUILD_PATH\\$DUB_TARGET_NAME.lib $PACKAGE_DIR\\lib"
        "copy /y $DUB_BUILD_PATH\\$DUB_TARGET_NAME.exp $PACKAGE_DIR\\lib"
    ],
-------