tests/fetchGitSubmodules: fix for newer Git

tests/fetchGitSubmodules.sh

@@ -14,6 +14,15 @@ subRepo=$TEST_ROOT/gitSubmodulesSub
 
 rm -rf ${rootRepo} ${subRepo} $TEST_HOME/.cache/nix
 
+# Submodules can't be fetched locally by default, which can cause
+# information leakage vulnerabilities, but for these tests our
+# submodule is intentionally local and it's all trusted, so we
+# disable this restriction. Setting it per repo is not sufficient, as
+# the repo-local config does not apply to the commands run from
+# outside the repos by Nix.
+export XDG_CONFIG_HOME=$TEST_HOME/.config
+git config --global protocol.file.allow always
+
 initGitRepo() {
     git init $1
     git -C $1 config user.email "foobar@example.com"