disable image push/slack notification for non main branch

[shivakunv]

Issue 1:
If the pipeline (workflow_dispatch: a manual run from a feature branch) pushes the base and driver images to the ghcr repo, the scheduled pipeline on main will treat that kernel version already published for base and driver image.
As a result, it will skip e2e tests and image publish for that kernel version for main branch.

Solution:
For manual runs via workflow_dispatch on a feature branch ( not main):

1. disable slack notifications: do not send slack for new-kernel or published image events.
2. disable docker push: do not push base or driver images to the registry.

This will restrict Slack and image pushes to main (e.g. scheduled runs or manual runs from main).
on feature branches, only build and test, no notifications and no pushes.
That keeps scheduled runs on main as the only ones that publish and notify.

issue 2:
file: tests/scripts/findkernelversion.sh :
Script set KERNEL_FLAVOR="${1}" but used suffix="${kernel_flavor}-${DIST}".
kernel_flavor is never set, so suffix became -${DIST} (e.g. -ubuntu22.04) instead of generic-ubuntu22.04.
When it works:
Artifact dirs happen to match the wrong pattern (e.g. only one flavor, or first match ).
Solution: suffix="${KERNEL_FLAVOR}-${DIST}"

Issue 3:

job: 
 Collect-e2e-test-matrix: artifact path

Script expected artifact files under ./matrix-values-artifacts/${artifact_name}/matrix_values_${dist}_${kernel}.json

With

    download-artifact:
    merge-multiple: false

when only one artifact matches it is extracted flat into ./matrix-values-artifacts/ (no artifact-name subdir), so the file is at ./matrix-values-artifacts/matrix_values_${dist}_${kernel}.json.

When it works:
Multiple matrix artifacts: download creates one subdir per artifact.

Solution: After checking the nested path, check the flat path: ./matrix-values-artifacts/matrix_values_${dist}_${kernel}.json, and use whichever exists so both single-artifact and multi-artifact work.

passed pipeline:
https://github.com/NVIDIA/gpu-driver-container/actions/runs/22710560690/job/65852046464

[rahulait]

LGTM