Avoid silent failure in export

Co-authored-by: Florian Sesser <florian@leastauthority.com>

Signed-off-by: Benoit Donneaux <benoit@leastauthority.com>

nixos/modules/update-deployment

@@ -79,7 +79,11 @@ ssh -o StrictHostKeyChecking=no "$(hostname).$(domainname)" ":"
 #
 # So instead, import our nixpkgs which forces it to be instantiated in the
 # store, then ask for its path, then set NIX_PATH to that.
-export NIX_PATH="nixpkgs=$(nix eval "(import ${CHECKOUT}/nixpkgs.nix { }).path")"
+
+# Two lines since 'export' masks 'set -e'
+# See https://mywiki.wooledge.org/BashFAQ/105#line-204
+NIX_PATH="nixpkgs=$(nix eval "(import ${CHECKOUT}/nixpkgs.nix { }).path")"
+export NIX_PATH
 
 # Attempt to update just this host.  Choose the morph grid definition matching
 # the grid we belong to and limit the morph deployment update to the host