Merge pull request #22649 from keszybz/symlink-enablement-yet-again-punish-me-harder

Fixups to the unit enablement logic

docs/ENVIRONMENT.md

@@ -43,6 +43,11 @@ All tools:
   debugging, in order to test generators and other code against specific kernel
   command lines.
 
+* `$SYSTEMD_OS_RELEASE` — if set, use this path instead of `/etc/os-release` or
+  `/usr/lib/os-release`. When operating under some root (e.g. `systemctl
+  --root=…`), the path is taken relative to the outside root. Only useful for
+  debugging.
+
 * `$SYSTEMD_FSTAB` — if set, use this path instead of `/etc/fstab`. Only useful
   for debugging.