nixos-rebuild-ng: tolerate nixpkgs being read-only

Classic nixos-rebuild ignores errors
when writing `.version-suffix` since
commit 365307ada1 .
Without this it is not possible to do
`nixos-rebuild -I nixpkgs=/path/to/read-only/nixpkgs ...`.
The commit at hand reconstructs the
classic behaviour for `nixos-rebuild-ng`.

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/services.py

@@ -372,4 +372,10 @@ def write_version_suffix(build_flags: Args) -> None:
     nixpkgs_path = nix.find_file("nixpkgs", build_flags)
     rev = nix.get_nixpkgs_rev(nixpkgs_path)
     if nixpkgs_path and rev:
-        (nixpkgs_path / ".version-suffix").write_text(rev)
+        try:
+            (nixpkgs_path / ".version-suffix").write_text(rev)
+        except OSError as error:
+            logger.debug(
+                "ignoring error while writing '.version-suffix' to nixpkgs: %s",
+                error,
+            )