This allows the user to configure systemd tmpfiles.d via
`environment.etc."tmpfiles.d/X.conf".text = "..."`, which after #93073
causes permission denied (with new X.conf):
```
ln: failed to create symbolic link '/nix/store/...-etc/etc/tmpfiles.d/X.conf': Permission denied
builder for '/nix/store/...-etc.drv' failed with exit code 1
```
or collision between environment.etc and systemd-default-tmpfiles
packages (with existing X.conf, such as tmp.conf):
```
duplicate entry tmpfiles.d/tmp.conf -> /nix/store/...-etc-tmp.conf
mismatched duplicate entry /nix/store/...-systemd-246/example/tmpfiles.d/tmp.conf <-> /nix/store/...-etc-tmp.conf
builder for '/nix/store/...-etc.drv' failed with exit code 1
```
Fixes#96755
1a68e21d47