fix: email digest user not found

[pateljannat]

Issue:

1. In an Email Digest, after clicking Send Now, the whole site used to become inaccessible and an error used to appear.

2. This used to happen because on clicking Send Now, the code was setting the recipient of email as the session user, send the email, and then reset the original user as session user.
3. For all this frappe.set_user function was used and this function clears the session after the user gets set. Hence the error.

Fix: Not changing the session user before sending the email.

[pateljannat]

closes #21337

[dawoodjee]

If we cannot use frappe.set_user() what's the right way to set permissions for a guest user's initiated action that inserts a record like Payment Entry for example?