formula: configure git/npm to ignore .brew_home

Library/Homebrew/formula.rb

@@ -2673,6 +2673,8 @@ class Formula
   def setup_home(home)
     # Don't let bazel write to tmp directories we don't control or clean.
     (home/".bazelrc").write "startup --output_user_root=#{home}/_bazel"
+    # Don't dirty the git tree for git clones.
+    (home/".gitignore").write "*"
   end
 
   # Returns a list of Dependency objects that are declared in the formula.

Library/Homebrew/language/node.rb

@@ -55,9 +55,6 @@ module Language
     sig { params(libexec: Pathname).returns(T::Array[String]) }
     def self.std_npm_install_args(libexec)
       setup_npm_environment
-      # tell npm to not install .brew_home by adding it to the .npmignore file
-      # (or creating a new one if no .npmignore file already exists)
-      open(".npmignore", "a") { |f| f.write("\n.brew_home\n") }
 
       pack = pack_for_installation