Add linux style path parsing for build_profile.json5

[janeoa]

Currently, in the docker/runner/signing-util when you do

uv run main.py ~/path-to-build-profile.json5

it expects the build-profile.json5 to have windows style C:\\path\to\certs, which I did not expect. So when I would run the main.py it would say File sign.sh written and create the sign.sh but key_base_name would be set to /home/username1 instead of actual value.

This PR tries to unify the parser by converting windows style to linux style and then parsing the linux style path.
I also included a test just in case, which can be run from docker/runner/signing-util using uv run python -m unittest

[Narfinger]

I don't think we need tests for this script.