Get demo.py running on MacOS

[englehardt]

A few notes:

• I was getting errors from mamba about not wanting to overwrite an existing environment. From the docs, I think we intend to, so I've added --force which will overwrite. I'm not sure if this is a platform difference?
• The firefox binary has changed names on MacOS
• I've migrated storage_watchdog.py over to python native code. Let's avoid subprocess calls out to system utilities as much as possible. MacOS has independent implementations of the OS utils, often with different options. I was tempted to install gnu-coreutils to fix this, but I'd rather not add another dependency when we can easily do the same calculation in python.

[englehardt]

The tests were failing because of the --force option I added. I see in #1088 we removed it intentionally. I probably just have an outdated version of mamba.

But it looking it up, it seems the mamba solver is now used by conda: https://github.com/conda-forge/miniforge?tab=readme-ov-file#whats-the-difference-between-mambaforge-and-miniforge. So we should update https://github.com/openwpm/OpenWPM?tab=readme-ov-file#pre-requisites and the install files to revert back to the latest miniforge.