Format .nix files

This does not include any automation for the release branch, but
is based on the configuration of https://github.com/NixOS/nix/pull/12349

    pre-commit run -a nixfmt-rfc-style

src/nix-env/buildenv.nix

@@ -8,13 +8,15 @@ derivation {
   inherit manifest;
 
   # !!! grmbl, need structured data for passing this in a clean way.
-  derivations =
-    map (d:
-      [ (d.meta.active or "true")
-        (d.meta.priority or 5)
-        (builtins.length d.outputs)
-      ] ++ map (output: builtins.getAttr output d) d.outputs)
-      derivations;
+  derivations = map (
+    d:
+    [
+      (d.meta.active or "true")
+      (d.meta.priority or 5)
+      (builtins.length d.outputs)
+    ]
+    ++ map (output: builtins.getAttr output d) d.outputs
+  ) derivations;
 
   # Building user environments remotely just causes huge amounts of
   # network traffic, so don't do that.