Fall back to the vendor os-release file when etc is absent

[OskarEichler]

Summary

A Linux installation containing only /usr/lib/os-release currently raises. Use it when /etc/os-release is absent; never merge the two files.

Reproduction and verification

Checked vendor-only, both-files, empty-etc and neither-file fixtures, without reading or altering system configuration. See the upstream format and precedence specification.

• External focused repro corpus: 4 checks pass against this isolated branch, under rbenv Ruby 4.0.6.
• Existing upstream suite: 36 examples, zero failures before and after the patch (RBENV_VERSION=4.0.6 rbenv exec rspec -Ilib -Ispec).
• Source-only change; no existing tests modified or new test files added, per the contributor's task constraint. The focused corpus lives outside the repository.
• Local verification was on macOS. Other Ruby versions and native operating systems were not executed. Upstream RuboCop configuration cannot load its legacy rubocop-lts1_8.yml, including after installing current rubocop-lts; lint status is not represented as passing.

Breaking-change notes

Existing /etc/os-release data, including an empty file, retains precedence. A previously rejected vendor-only installation now succeeds; the missing-file error identifies the attempted path.