Automate reinstallation

Manually picked from starship/starship#5845.

getting-started.md

@@ -326,26 +326,27 @@ See [`MISE_FISH_AUTO_ACTIVATE=1`](/configuration#mise_fish_auto_activate1) for m
 
 #### Nushell
 
-Since Nu does [not support `eval`](https://www.nushell.sh/book/how_nushell_code_gets_run.html#eval-function) you must save the activation script:
+Since Nu does [not support `eval`](https://www.nushell.sh/book/how_nushell_code_gets_run.html#eval-function) the initialization script is saved in `env.nu`:
 
 ```nushell
+'
 let mise_path = $nu.default-config-dir | path join scripts mise.nu
-^mise activate nu | save $mise_path --force
+if not (
+  $mise_path | path exists
+) or (
+  (date now) - (ls $mise_path).modified.0 > 1day
+) {
+  ^mise activate nu | save $mise_path --force
+}' | save $nu.env-path --append
 "\nuse mise.nu" | save $nu.config-path --append
 ```
 
-If you prefer to keep your dotfiles clean you can save it to another directory then update `$env.NU_LIB_DIRS`:
+If you prefer to keep your dotfiles clean you can save it to a different directory then update `$env.NU_LIB_DIRS`:
 
 ```nushell
 "\n$env.NU_LIB_DIRS ++= ($mise_path | path dirname | to nuon)" | save $nu.env-path --append
 ```
 
-You can update the generated script at any time by re-running:
-
-```nushell
-^mise activate nu | save $mise_path --force
-```
-
 #### Xonsh
 
 Since `.xsh` files are [not compiled](https://github.com/xonsh/xonsh/issues/3953) you may shave a bit off startup time by using a pure Python import: add the code below to, for example, `~/.config/xonsh/mise.py` config file and `import mise` it in `~/.config/xonsh/rc.xsh`: