Make the secrets commands inline aware

Rather than redirecting the global $stdout, which is not never clever in
a threaded program, we'll make the secrets commands aware they are
being inlined, so they return the value instead of printing it.

Additionally we no longer need to interrupt the parent process on error
as we've inlined the command - exit 1 is enough.

test/secrets/dotenv_inline_command_substitution_test.rb

@@ -2,7 +2,7 @@ require "test_helper"
 
 class SecretsInlineCommandSubstitution < SecretAdapterTestCase
   test "inlines kamal secrets commands" do
-    Kamal::Cli::Main.expects(:start).with { |command| puts "results"; command == [ "secrets", "fetch", "..." ] }
+    Kamal::Cli::Main.expects(:start).with { |command| command == [ "secrets", "fetch", "...", "--inline" ] }.returns("results")
     substituted = Kamal::Secrets::Dotenv::InlineCommandSubstitution.call("FOO=$(kamal secrets fetch ...)", nil, overwrite: false)
     assert_equal "FOO=results", substituted
   end