Fix README download links and Ubuntu artifact naming for .deb packages

[Copilot]

README download links pointed to 0.3.7 (incompatible with current OBS), and .deb packages were missing from 0.4.0 release due to undefined ${{ matrix.os }} in Ubuntu build job.

Changes

README.md

• Updated download links from 0.3.7 to 0.4.0 release assets
• Linux link temporarily points to source tarball until .deb available

.github/workflows/build-project.yaml

• Fixed Ubuntu artifact naming by replacing undefined ${{ matrix.os }} with ubuntu-24.04

The ubuntu-build job had no matrix defined, causing artifacts to be named obs-urlsource-0.4.0--x86_64-* instead of obs-urlsource-0.4.0-ubuntu-24.04-x86_64-*. The release workflow in push.yaml expects the latter pattern:

variants=(
  'ubuntu-24.04-x86_64;tar.xz|deb|ddeb'
)

Future releases will properly include .deb packages.

Not Addressed

• Issue Add option to output via text source #3 (404 link in release notes): Requires manual edit to published release

Original prompt

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This section details on the original issue you should resolve

<issue_title>A bunch of problems during installation 0.4.0 release</issue_title>
<issue_description>I've noticed some problems during installation of 0.4.0 release on Ubuntu. They are not looking as big to create a separate issue for each.

1. Download links in README header are leading to 0.3.7 release. Which is not compatible with current version of OBS studio.
2. An installation instruction for Ubuntu gives wrong shell command. It should be $ sudo cp -R release/RelWithDebInfo/lib/* /usr/lib/
3. A link to README in Installation section of release 0.4.0 leads to 404 page.
4. There are no .deb packages in 0.4.0 release. Not every Linux user prefers to build C projects locally. Therefore, it might be better to keep providing a .deb package option for them.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes A bunch of problems during installation 0.4.0 release #143

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[royshil]

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