[CI] Fix test_macos with recent bash for nix-shell (#10480)

* [CI] Initialize nix environment in prepare_build

* [CI] Install recent bash for nix-shell on macos

bin/ci

@@ -149,6 +149,14 @@ prepare_build() {
   on_osx brew update --preinstall
   on_osx brew bundle --no-lock
 
+  # Install a recent bash version for nix-shell.
+  # macos ships with an ancient one.
+  if [ `uname` = "Darwin" ]; then
+    on_nix_shell "brew install bash"
+  fi
+  # initialize nix environment
+  on_nix_shell nix-shell
+
   # Note: brew link --force might show:
   #   Warning: Refusing to link macOS-provided software: llvm
   #