env: fix missing hscloud_nixos export

Change-Id: I1887a06908e8b50926288d4cd9c9a820dd795ae0
diff --git a/env.sh b/env.sh
index 374df77..27cf34f 100644
--- a/env.sh
+++ b/env.sh
@@ -18,7 +18,7 @@
 
 # Detect NixOS
 if [ -d /nix ] && [ ! -f /lib/ld-linux.so.2 ]; then
-    hscloud_nixos=true
+    export hscloud_nixos=true
 fi
 
 gpg-unlock() {