Work around regression of treefmt that only formats first file path

See numtide/treefmt-nix#253.

README.md

@@ -90,5 +90,5 @@ The following tools are integrated:
 - [**treefmt**](https://github.com/numtide/treefmt)
 
   ```bash
-  treefmt --fail-on-change --no-cache *.bzl *.cjs *.clj *.cljc *.cljs *.cljx *.css *.html *.js *.json *.json5 *.jsonnet *.jsx *.libsonnet *.md *.mdx *.mjs *.nix *.py *.pyi *.rb *.scss *.sh *.tf *.toml *.ts *.tsx *.vue *.yaml *.yml BUILD.bazel Vagrantfile
+  treefmt --fail-on-change --no-cache --walk filesystem *.bzl *.cjs *.clj *.cljc *.cljs *.cljx *.css *.html *.js *.json *.json5 *.jsonnet *.jsx *.libsonnet *.md *.mdx *.mjs *.nix *.py *.pyi *.rb *.scss *.sh *.tf *.toml *.ts *.tsx *.vue *.yaml *.yml BUILD.bazel Vagrantfile
   ```

flake.nix

@@ -80,6 +80,8 @@
               "${treefmtEval.config.build.wrapper}/bin/treefmt"
               "--fail-on-change"
               "--no-cache"
+              "--walk"
+              "filesystem"
               # `"--"` would break treefmt as it seems to append `--tree-root`.
             ];
             file_patterns = let

test/expected_calls.txt

@@ -6,4 +6,4 @@ Would run: /…/jsonnet-lint -- jsonnet_lint.jsonnet jsonnetfmt.jsonnet
 Would run: /…/pylint -- black.py isort.py pylint.py
 Would run: /…/shellcheck -- shellcheck.sh shfmt.sh
 Would run: /…/stylelint -- stylelint.css
-Would run: /…/treefmt --fail-on-change --no-cache Vagrantfile alejandra.nix black.py buildifier.bzl cljfmt.clj isort.py jsonnet_lint.jsonnet jsonnetfmt.jsonnet prettier.js prettier.toml pylint.py rufo.rb shellcheck.sh shfmt.sh statix.nix stylelint.css terraform.tf
+Would run: /…/treefmt --fail-on-change --no-cache --walk filesystem Vagrantfile alejandra.nix black.py buildifier.bzl cljfmt.clj isort.py jsonnet_lint.jsonnet jsonnetfmt.jsonnet prettier.js prettier.toml pylint.py rufo.rb shellcheck.sh shfmt.sh statix.nix stylelint.css terraform.tf