custom /etc/os-release

Currently we hardcode the contents of [/etc/os-release](https://github.com/bottlerocket-os/twoliter/blob/5c4c66055506e171600d077ac652f5bfe50316b1/twoliter/embedded/rpm2img#L373) based on build-related variables.

That locks downstream builds into specific outputs that they may not want; for example, `PRETTY_NAME` will always be something like "Bottlerocket x.y.z" even if the version isn't meaningful.

What I'd suggest instead is that:
1. if no `/etc/os-release` exists in the sys-root, we write the existing `os-release` as a file with unexpanded variables
2. then we apply `envsubst` to `/etc/os-release` in a second pass to fill in variables we want to replace

The URL fields should also be parameterized so that variable substitution works there too.

Read twoliter/embedded/rpm2img around the linked /etc/os-release generation. Check how the sys-root file is handled, then verify that an existing file is preserved, a missing file gets the unexpanded template, and variable substitution also covers the URL fields.

Written by the indexing model from the issue text.