Fix debian PackageCloud upload

[viktorerlingsson]

The commit c979e4c changed the build output directory structure from builds/ubuntu/jammy/.deb to builds/linux_amd64/.deb. The PackageCloud upload script was still trying to extract distribution info from the file path, resulting in empty DIST_ID values and 422 errors.

This fix extracts the distribution ID and version codename from the matrix.os variable instead (e.g., "ubuntu-jammy" → "ubuntu" + "jammy"), matching the pattern already used successfully in the RPM section.

Fixes #236

[fretb]

FYI, I think this broke some packages for Debian systems, see #236 (comment).

Looks like the Bullseye package for instance, was not built on a Bullseye system.

~# /usr/bin/apt-get install amqproxy
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 amqproxy : Depends: libc6 (>= 2.38) but 2.31-13+deb11u13 is to be installed
            Depends: libssl3t64 (>= 3.0.0) but it is not installable
E: Unable to correct problems, you have held broken packages.