scripts: align --context path with user-specified bisection_context.yaml

Update execute_bisection.sh to align with the user-specified
bisection_context.yaml file by accepting --context as an explicit
command-line argument.

Fixes: 2f7460b0 ("scripts: add execute_bisection.sh for git bisect-based regression detection")
Signed-off-by: Aishwarya TCV <aishwarya.tcv@arm.com>