ktest.pl: Allow command option -D to override temp variables

Currently -D only updates the persistent options that are defined with
"=". Allow it to also override all temp variables that are defined with
":=".

 ktest.pl -D 'USE_TEMP_DIR:=1' -D 'TEST_TYPE[2]=build' config

Cc: "John Warthog9 Hawley" <warthog9@kernel.org>
Cc: Dhaval Giani <dhaval.giani@gmail.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/20250718202053.399653933@kernel.org


Signed-off-by: Steven Rostedt <rostedt@goodmis.org>