fix(nu): continuation prompt not being displayed correctly (#5851)

Resolve #5847 by initializing `$env.STARSHIP_SHELL` before calling `starship`

Most minimal change possible. Other simplification are possible but not know to be
backwards compatible. Also considered `with-env` or `STARSHIP_SHELL=nu starship` but
they'd be worse. Happy to reformat indentation if desired.

src/init/starship.nu

@@ -2,8 +2,7 @@
 # - overlay which can be loaded with `overlay use starship.nu`
 # - module which can be used with `use starship.nu`
 # - script which can be used with `source starship.nu`
-export-env { load-env {
-    STARSHIP_SHELL: "nu"
+export-env { $env.STARSHIP_SHELL = "nu"; load-env {
     STARSHIP_SESSION_KEY: (random chars -l 16)
     PROMPT_MULTILINE_INDICATOR: (
         ^::STARSHIP:: prompt --continuation