CI(run-python-test-set): allow to skip missing compatibility snapshot (#9365)

## Problem
Action `run-python-test-set` fails if it is not used for `regress_tests`
on release PR, because it expects
`test_compatibility.py::test_create_snapshot` to generate a snapshot,
and the test exists only in `regress_tests` suite.
For example, in https://github.com/neondatabase/neon/pull/9291
[`test-postgres-client-libs`](https://github.com/neondatabase/neon/actions/runs/11209615321/job/31155111544)
job failed.

## Summary of changes
- Add `skip-if-does-not-exist` input to `.github/actions/upload` action
(the same way we do for `.github/actions/download`)
- Set `skip-if-does-not-exist=true` for "Upload compatibility snapshot"
step in `run-python-test-set` action

.github/actions/run-python-test-set/action.yml

@@ -218,6 +218,9 @@ runs:
         name: compatibility-snapshot-${{ runner.arch }}-${{ inputs.build_type }}-pg${{ inputs.pg_version }}
         # Directory is created by test_compatibility.py::test_create_snapshot, keep the path in sync with the test
         path: /tmp/test_output/compatibility_snapshot_pg${{ inputs.pg_version }}/
+        # The lack of compatibility snapshot shouldn't fail the job
+        # (for example if we didn't run the test for non build-and-test workflow)
+        skip-if-does-not-exist: true
 
     - name: Upload test results
       if: ${{ !cancelled() }}