nvm: add nvm-exec.

Closes Homebrew/homebrew#38691.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/nvm.rb

@@ -4,9 +4,19 @@ class Nvm < Formula
   sha256 "0583b948f519efaa8fbabd630fae86e2d919c32173fba9ed0db4fcba4640de97"
   head "https://github.com/creationix/nvm.git"
 
+  resource "nvm-exec" do
+    url "https://raw.githubusercontent.com/creationix/nvm/v0.24.1/nvm-exec"
+    sha256 "915118c666c6f3e03ce4a4545038ecae691c7c496579c614f4d32200a180924c"
+  end
+
   def install
     prefix.install "nvm.sh"
     bash_completion.install "bash_completion" => "nvm"
+
+    resource("nvm-exec").stage do
+      prefix.install("nvm-exec")
+      chmod 0755, "#{prefix}/nvm-exec"
+    end
   end
 
   def caveats; <<-EOS.undent
@@ -14,6 +24,10 @@ class Nvm < Formula
 
         mkdir ~/.nvm
 
+      Copy nvm-exec to NVM's working directory
+
+        cp $(brew --prefix nvm)/nvm-exec ~/.nvm/
+
       Add the following to $HOME/.bashrc, $HOME/.zshrc, or your shell's
       equivalent configuration file: