Set $HOME to the user's home directory in sync scripts

Makes sure git can find the user's git configuration.

mkosi/__init__.py

@@ -435,6 +435,7 @@ def run_sync_scripts(context: Context) -> None:
         MKOSI_UID=str(INVOKING_USER.uid),
         MKOSI_GID=str(INVOKING_USER.gid),
         MKOSI_CONFIG="/work/config.json",
+        HOME=INVOKING_USER.home(),
         CACHED=one_zero(have_cache(context.config)),
     )