nix: Safer shellHook

Check if starship is installed before invoking. (It should be, but let's
be careful.)

shell.nix

@@ -10,6 +10,6 @@ pkgs.mkShellNoCC {
     typescript-language-server
   ];
   shellHook = ''
-    eval "$(starship init bash)"
+    which -s starship >/dev/null 2>&1 && eval "$(starship init bash)"
   '';
 }