Fix mastodon:setup choking on env variables containing '%' (#13940)

Fix a regression introduced in #13928, caused by TTY::Command building
shell commands by chaining string substitutions.

Ditch TTY::Command and use system instead (both do shell out).

Gemfile.lock

@@ -623,8 +623,6 @@ GEM
     thwait (0.1.0)
     tilt (2.0.10)
     tty-color (0.5.1)
-    tty-command (0.9.0)
-      pastel (~> 0.7.0)
     tty-cursor (0.7.1)
     tty-prompt (0.21.0)
       necromancer (~> 0.5.0)
@@ -792,7 +790,6 @@ DEPENDENCIES
   strong_migrations (~> 0.6)
   thor (~> 0.20)
   thwait (~> 0.1.0)
-  tty-command (~> 0.9)
   tty-prompt (~> 0.21)
   twitter-text (~> 1.14)
   tzinfo-data (~> 1.2020)