nushell: fix get -i deprecation (#7490)

Since https://github.com/nushell/nushell/pull/16007, the recommended
flag to avoid erroring on missing fields is `--optional`. To avoid
compatibility issues, the builtin optional access syntax is used
instead, which is backwards-compatible.

modules/programs/direnv.nix

@@ -193,8 +193,8 @@ in
                           }
                         }
                         | merge ($env.ENV_CONVERSIONS? | default {})
-                        | get -i ([[value, optional, insensitive]; [$key, false, true]] | into cell-path)
-                        | get -i from_string
+                        | get ([[value, optional, insensitive]; [$key, true, true]] | into cell-path)
+                        | get from_string?
                         | if ($in | is-empty) { {|x| $x} } else { $in }
                     ) $value
                     return [ $key $value ]