Use checkout@v1 in susemanager-sls-compatible-python26-syntax workflow

[HoussemNasri]

What does this PR change?

Checkout@v1 is the only version compatible with python 2.6. Using a higher version leads to workflows failing when checking out the repo, see https://github.com/uyuni-project/uyuni/actions/runs/6427883155/job/17454183064?pr=7466

Refs #7498

GUI diff

No difference.

• DONE

Documentation

• No documentation needed

• DONE

Test coverage

• No tests: already covered

• DONE

Links

• DONE

Changelogs

Make sure the changelogs entries you are adding are compliant with https://github.com/uyuni-project/uyuni/wiki/Contributing#changelogs and https://github.com/uyuni-project/uyuni/wiki/Contributing#uyuni-projectuyuni-repository

If you don't need a changelog check, please mark this checkbox:

• No changelog needed

If you uncheck the checkbox after the PR is created, you will need to re-run changelog_test (see below)

Re-run a test

If you need to re-run a test, please mark the related checkbox, it will be unchecked automatically once it has re-run:

• Re-run test "changelog_test"
• Re-run test "backend_unittests_pgsql"
• Re-run test "java_pgsql_tests"
• Re-run test "schema_migration_test_pgsql"
• Re-run test "susemanager_unittests"
• Re-run test "javascript_lint"
• Re-run test "spacecmd_unittests"

[meaksh]

LGTM, thanks!