fix: update rugged gem version to 1.9.6 on alpine; switch to system package on ubuntu

[rwaffen]

Short description

ubuntu

rugged needs libgit2 and both need to be on the same version.
ubuntu only has libgit2 1.9.1, so we cannot install rugged 1.9.6.

hack: install ruby-rugged system package which is build against the system libgit2 version.

alpine

The build stays the same.

Fixes #161

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[rwaffen]

for refernce: https://launchpad.net/ubuntu/+source/libgit2