Explicity allow use of FetchContent for Solidity

Since Homebrew/brew#17075, CMake `FetchContent` is disabled by default.
This made the Solidity formula to crash: https://github.com/Homebrew/homebrew-core/actions/runs/9175554982/job/25228895627?pr=172324.
This commit explicitly enables it back.

See: Homebrew/brew#17187 (comment) for context.

Formula/s/solidity.rb

@@ -28,7 +28,7 @@ class Solidity < Formula
 
   def install
     mkdir "build" do
-      system "cmake", "..", *std_cmake_args
+      system "cmake", "..", *std_cmake_args, "-DHOMEBREW_ALLOW_FETCHCONTENT=ON"
       system "make", "install"
     end
   end