rust: don't rerun git submodule for HEAD builds

Fixes Homebrew/homebrew#48980.

Closes Homebrew/homebrew#49370.

Signed-off-by: Alex Dunn <dunn.alex@gmail.com>

Formula/rust.rb

@@ -49,6 +49,12 @@ class Rust < Formula
   end
 
   def install
+    # Because we copy the source tree to a temporary build directory,
+    # the absolute paths written to the `gitdir` files of the
+    # submodules are no longer accurate, and running `git submodule
+    # update` during the configure step fails.
+    ENV["CFG_DISABLE_MANAGE_SUBMODULES"] = "1" if build.head?
+
     args = ["--prefix=#{prefix}"]
     args << "--disable-rpath" if build.head?
     args << "--enable-clang" if ENV.compiler == :clang