Fix uninitialized readFromStdIn variable

This was causing random failures in tests/ca/substitute.ca: 'nix copy
--file ./content-addressed.nix' wouldn't get the default installable
'.' applied in InstallablesCommand::load(), so it would do nothing.

src/libcmd/command.hh

@@ -128,7 +128,7 @@ struct InstallablesCommand : virtual Args, SourceExprCommand
 
     virtual bool useDefaultInstallables() { return true; }
 
-    bool readFromStdIn;
+    bool readFromStdIn = false;
 
     std::vector<std::string> getFlakesForCompletion() override;