Prevent missing github authentication directories from stopping downloading PDKs

[sizeru]

When running volare with the following command (following a fresh install):

./venv/bin/volare enable --pdk sky130

I get the following error:

[Errno 20] Not a directory: 'gh'

If github authentication information cannot be found, this causes the pdk to never be downloaded. The following fix catches this error.

[donn]

Could I ask you to refile against the maintained fork https://github.com/fossi-foundation/ciel please?