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

tests/repl-completion.nix

@@ -1,40 +1,45 @@
-{ runCommand, nix, expect }:
+{
+  runCommand,
+  nix,
+  expect,
+}:
 
 # We only use expect when necessary, e.g. for testing tab completion in nix repl.
 # See also tests/functional/repl.sh
 
-runCommand "repl-completion" {
-  nativeBuildInputs = [
-    expect
-    nix
-  ];
-  expectScript = ''
-    # Regression https://github.com/NixOS/nix/pull/10778
-    spawn nix repl --offline --extra-experimental-features nix-command
-    expect "nix-repl>"
-    send "foo = import ./does-not-exist.nix\n"
-    expect "nix-repl>"
-    send "foo.\t"
-    expect {
-      "nix-repl>" {
-        puts "Got another prompt. Good."
+runCommand "repl-completion"
+  {
+    nativeBuildInputs = [
+      expect
+      nix
+    ];
+    expectScript = ''
+      # Regression https://github.com/NixOS/nix/pull/10778
+      spawn nix repl --offline --extra-experimental-features nix-command
+      expect "nix-repl>"
+      send "foo = import ./does-not-exist.nix\n"
+      expect "nix-repl>"
+      send "foo.\t"
+      expect {
+        "nix-repl>" {
+          puts "Got another prompt. Good."
+        }
+        eof {
+          puts "Got EOF. Bad."
+          exit 1
+        }
       }
-      eof {
-        puts "Got EOF. Bad."
-        exit 1
-      }
-    }
-    exit 0
-  '';
-  passAsFile = [ "expectScript" ];
-}
-''
-  export NIX_STORE=$TMPDIR/store
-  export NIX_STATE_DIR=$TMPDIR/state
-  export HOME=$TMPDIR/home
-  mkdir $HOME
+      exit 0
+    '';
+    passAsFile = [ "expectScript" ];
+  }
+  ''
+    export NIX_STORE=$TMPDIR/store
+    export NIX_STATE_DIR=$TMPDIR/state
+    export HOME=$TMPDIR/home
+    mkdir $HOME
 
-  nix-store --init
-  expect $expectScriptPath
-  touch $out
-''
+    nix-store --init
+    expect $expectScriptPath
+    touch $out
+  ''