Access workdir defined on command line from within Snakefile
You could query Python for the working directory within the Snakefile, since evidently Snakemake changes the actual working directory of the process to the one specified by --directory.    For example, using a stub Snakefile with just these two lines:

import os
print("Current working directory: " + os.getcwd())


And on the command line:

$ snakemake --directory working-directory
Creating specified working directory working-directory.
Current working directory: /home/jesse/dev/snakemake-wd/working-directory
Building DAG of jobs...
Nothing to be done.
Complete log: /home/jesse/dev/snakemake-wd/working-directory/.snakemake/log/2019-08-02T142009.349338.snakemake.log