Fix CI build-buster job

[legoktm]

Description

There's some bug in which building from a sdist tarball doesn't work.
Given that we wanted to move away from that anyways, switch to just
specifying the path to the directory instead.

Needs freedomofpress/securedrop-builder#332 to work.

✨ @creviera for finding out the difference in behavior between the tarball and the directory builds!

Test Plan

• CI passes (once s-d-p patch is merged)

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, the AppArmor profile may need to be updated. Please check as applicable:

• I have updated the AppArmor profile
• No update to the AppArmor profile is required for these changes
• I don't know and would appreciate guidance

If these changes modify the database schema, you should include a database migration. Please check as applicable:

• I have written a migration and upgraded a test database based on main and confirmed that the migration applies cleanly
• I have written a migration but have not upgraded a test database based on main and would like the reviewer to do so
• I need help writing a database migration
• No database schema changes are needed

[gonzalo-bulnes]

I only touched the commit to sign it. 🔏

[gonzalo-bulnes]

Using the directory instead of the tarball as PKG_PATH is equivalent in this case. We've discussed this option with @creviera, and making the switch will fix the main CI build, as well as unlock work in progress.

We may want to evaluate the option of specifying a tarball as a next step, but that doesn't need to block ongoing work.

Thanks @legoktm!