root: add .envrc

Change-Id: I969b8d32157e408d09bc2f32b8450581d5eb9c37
Reviewed-on: https://review.monogon.dev/c/monogon/+/4413
Reviewed-by: Leopold Schabel <leo@monogon.tech>
Tested-by: Jenkins CI
diff --git a/.envrc b/.envrc
new file mode 100644
index 0000000..1d953f4
--- /dev/null
+++ b/.envrc
@@ -0,0 +1 @@
+use nix