fix: preserve local direnv settings (#26122)

## Which issue does this PR close?

Closes #26101 , which was introduced in
#26005. cc @kumarUjjawal
@kosiew

## What changes are included in this PR?

Load an ignored .envrc.local after the shared Nix environment so
contributors can keep machine-specific settings without modifying
tracked files.

Document the Nix, direnv, and local override workflow in the development
environment guide.

Following recommendation from direnv:
https://github.com/direnv/direnv/blob/e24ea74873aff78d5e371c85061dc7fafdeedd5a/README.md#quick-demo

## Are there any user-facing changes?

No.

Author: VVKot

.envrc

@@ -1 +1,5 @@
-use flake
+source_up_if_exists
+if command -v nix >/dev/null 2>&1; then
+  use flake
+fi
+source_env_if_exists .envrc.local

.gitignore

@@ -72,6 +72,7 @@ datafusion/core/benches/data/*
 # Generated Nix environment hooks
 
 .direnv/
+.envrc.local
 
 # rat
 filtered_rat.txt

docs/source/contributor-guide/development_environment.md

@@ -52,6 +52,21 @@ Notes:
 - `protoc` is required to compile DataFusion from source.
 - Some tests and examples rely on git submodule data being present locally.
 
+## Nix and direnv
+
+The repository's `flake.nix` provides a Nix development shell. Enter it directly
+with `nix develop`, or install and configure [direnv](https://direnv.net/) and run
+`direnv allow` from the repository root to load it automatically.
+
+The repository's `.envrc` first loads the nearest ancestor `.envrc`, if one
+exists, and then loads the shared Nix environment when Nix is installed. Put
+project-specific direnv settings in `.envrc.local`. This file is ignored by Git
+and loaded last so that local settings take precedence.
+
+If you previously kept personal settings in the repository's `.envrc`, move
+only those settings to `.envrc.local`. Do not copy the shared `.envrc`, as doing
+so would load the Nix environment twice.
+
 ## Windows Setup
 
 ```shell