Use dummy input variables during `Scan` rewrites

[brandonwillard]

This PR (potentially) addresses some Scan issues that appear when rewrites use nominal variables too liberally.

At the moment, this is just a draft for testing purposes.

[codecov[bot]]

Codecov Report

Merging #1145 (324c564) into main (d7fb940) will increase coverage by 0.00%. The diff coverage is 45.45%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1145   +/-   ##
=======================================
  Coverage   79.35%   79.35%           
=======================================
  Files         161      161           
  Lines       48260    48262    +2     
  Branches    10955    10955           
=======================================
+ Hits        38295    38297    +2     
  Misses       7453     7453           
  Partials     2512     2512           

Impacted Files	Coverage Δ
aesara/scan/op.py	85.48% <0.00%> (ø)
aesara/scan/rewriting.py	79.07% <47.61%> (+0.03%)	:arrow_up: