gifn: add a Nix expression for running the Perl test suite

Change-Id: Iea891bbbf383b7ea2ab4b185a84ebcded4521efe
diff --git a/tools/git-if-needed/.editorconfig b/tools/git-if-needed/.editorconfig
index e63f0ca..26fbee5 100644
--- a/tools/git-if-needed/.editorconfig
+++ b/tools/git-if-needed/.editorconfig
@@ -7,6 +7,10 @@
 insert_final_newline = true
 charset = utf-8
 
+[*.nix]
+indent_style = space
+indent_size = 2
+
 # Mainly for the multi-line "Description: ..." header.
 [*.patch]
 indent_style = space