CI: Fix schema_migration_reportdb_test_pgsql workflow

[nodeg]

What does this PR change?

Adds the explicit Ruby version to the schema_migration_reportdb_test_pgsql workflow to fix:

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes
• DONE

Test coverage

• Cucumber tests were edited
• DONE

Links

Issue(s): #
Ports(s): # add downstream PR(s), if any

• 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_lint_checkstyle"
• Re-run test "java_pgsql_tests"
• Re-run test "ruby_rubocop"
• Re-run test "schema_migration_test_pgsql"
• Re-run test "susemanager_unittests"
• Re-run test "javascript_lint"
• Re-run test "spacecmd_unittests"

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