added shell prefix for nix shells and some nix changes

.config/oh-my-posh/zen.toml

@@ -40,6 +40,12 @@ template = '''
   foreground = 'yellow'
   template = '{{ if .Env.VIRTUAL_ENV }}🐍 venv <grey>|</> {{ end }}'
 
+	[[blocks.segments]] # Nix shell indicator
+  type = 'text'
+  style = 'plain'
+  foreground = 'blue'
+  template = '{{ if .Env.IN_NIX_SHELL }} nsh <grey>|</> {{ end }}'
+
   [[blocks.segments]]
     template = '{{ .Path }}'
     foreground = 'blue'