Implement maintainer feedback

Reinitializes the script at most once every day. This is necessary because doing so every
time could lead to thousands of concurrent writes calling `| par-each { nu -c }`.

docs/README.md

@@ -137,11 +137,18 @@ description: Starship is the minimal, blazing fast, and extremely customizable p
 
    #### Nushell
 
-   Since Nu does [not support `eval`](https://www.nushell.sh/book/how_nushell_code_gets_run.html#eval-function) you must save the initialization 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 starship_path = $nu.default-config-dir | path join scripts starship.nu
-   starship init nu | save $starship_path --force
+   if not (
+     $starship_path | path exists
+   ) or (
+     (date now) - (ls $starship_path).modified.0 > 1day
+   ) {
+     starship init nu | save $starship_path --force
+   }' | save $nu.env-path --append
    "\nuse starship.nu" | save $nu.config-path --append
    ```
 
@@ -151,12 +158,6 @@ description: Starship is the minimal, blazing fast, and extremely customizable p
    "\n$env.NU_LIB_DIRS ++= ($starship_path | path dirname | to nuon)" | save $nu.env-path --append
    ```
 
-   You can update the generated script at any time by re-running:
-
-   ```nushell
-   starship init nu | save $starship_path --force
-   ```
-
    #### Xonsh
 
    Add the following to the end of `~/.xonshrc`: