install: also configure ~/.zshenv

The default login shell for users on macOS 10.15 changed from bash to
zsh.  So while generally nonstandard we need to configure it to make nix
function out of the box on macOS.

scripts/install-nix-from-closure.sh

@@ -186,6 +186,17 @@ if [ -z "$NIX_INSTALLER_NO_MODIFY_PROFILE" ]; then
             break
         fi
     done
+    for i in .zshenv .zshrc; do
+        fn="$HOME/$i"
+        if [ -w "$fn" ]; then
+            if ! grep -q "$p" "$fn"; then
+                echo "modifying $fn..." >&2
+                echo "if [ -e $p ]; then . $p; fi # added by Nix installer" >> "$fn"
+            fi
+            added=1
+            break
+        fi
+    done
 fi
 
 if [ -z "$added" ]; then