fix(powershell): fix session key syntax error (starship#1697)

Author: Tilmann Meyer

src/init/starship.ps1

@@ -56,4 +56,4 @@ function global:prompt {
 $ENV:STARSHIP_SHELL = "powershell"
 
 # Set up the session key that will be used to store logs
-$ENV:STARSHIP_SESSION_KEY = $(::STARSHIP:: session)
+$ENV:STARSHIP_SESSION_KEY = (& ::STARSHIP:: session)