scripts: source --context path from .env for bisection_context.yaml

Update execute_bisection.sh to align with the user-specified
bisection_context.yaml file by sourcing the context path from an
environment file (.env).

This simplifies the script interface and ensures that all key input
values (context, kernel, modules, gitsha) can be consistently passed
via a single environment file, improving clarity and making the
invocation from GitLab CI cleaner and more robust.

Fixes: 2f7460b0 ("scripts: add execute_bisection.sh for git bisect-based regression detection")

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