Fix scan_checkpoints with sequences

[ricardoV94]

Description

Found this by accident while working with Join and trying to understand the view flag (see #753)

This was completely broken for sequences, but the fix is simple enough, so I took a stab at it.

Related Issue

• Related to Get rid of join view flag #753

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.81%. Comparing base (b8e26cd) to head (e5ff90e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #754   +/-   ##
=======================================
  Coverage   80.81%   80.81%           
=======================================
  Files         162      162           
  Lines       46808    46817    +9     
  Branches    11452    11453    +1     
=======================================
+ Hits        37826    37835    +9     
+ Misses       6734     6733    -1     
- Partials     2248     2249    +1     

Files	Coverage Δ
pytensor/scan/checkpoints.py	76.47% <100.00%> (+5.04%)	⬆️

... and 1 file with indirect coverage changes