Set BOOST_SRC environment variable

This makes the build process faster by allowing Antora extensions to fall back to this path to find the Boost source files to generate the reference pages.

ci_boost_release.py

@@ -624,6 +624,11 @@ def command_build(self):
         else:
             checkout_branch = "develop"
 
+        # Set the environment variable BOOST_SRC_DIR to the root directory
+        # so that the following scripts can find the boost source code
+        # instead of downloading it again.
+        os.environ["BOOST_SRC_DIR"] = self.root_dir
+
         # Call antora project main script
         os.chdir(self.build_dir)
         antora_dir = os.path.join(self.build_dir, "antora")