activation-script: default to skipping 'sanity' checks on darwin

this defaults to not checking that the `$HOME` env variable matches the
nix config entry for that user.

i think it's a safe assertion to make that given nix-darwin can't be
used for from-first-boot host provisioning, a lot less people manage
their user accounts with it. these "sanity" checks depend on the user in
question being a fully managed user, which is otherwise unnecessary and
caused some surprise breakages on home-manager updates.

modules/home-environment.nix

@@ -711,6 +711,8 @@ in
           set -eu
           set -o pipefail
 
+          : ''${SKIP_SANITY_CHECKS:=${ if pkgs.stdenvNoCC.isDarwin then "true" else "" }}
+
           cd $HOME
 
           export PATH="${activationBinPaths}"
@@ -718,7 +720,7 @@ in
 
           ${builtins.readFile ./lib-bash/activation-init.sh}
 
-          if [[ ! -v SKIP_SANITY_CHECKS ]]; then
+          if [[ -z SKIP_SANITY_CHECKS ]]; then
             checkUsername ${escapeShellArg config.home.username}
             checkHomeDirectory ${escapeShellArg config.home.homeDirectory}
           fi