Install the emsdk in toolchain directory to have CI cache it correctly.

kiwixbuild/dependencies/emsdk.py

@@ -23,7 +23,7 @@ def source_dir(self):
     class Builder(Builder):
         @property
         def install_path(self):
-            return self.build_path
+            return pj(self.buildEnv.toolchain_dir, self.target.full_name())
 
         def _copy_source(self, context):
             context.try_skip(self.build_path)