tcl.mkTclDerivation: fix exec prefix

Shell variables are not expanded, so the old value was just the literal
string `${out}`.

Fixes #139566

pkgs/development/interpreters/tcl/mk-tcl-derivation.nix

···
   defaultTclPkgConfigureFlags = [
     "--with-tcl=${tcl}/lib"
     "--with-tclinclude=${tcl}/include"
-    "--exec-prefix=\${out}"
+    "--exec-prefix=${placeholder "out"}"
   ];
 
   self = (stdenv.mkDerivation ((builtins.removeAttrs attrs [