Fix downgrade test artifact upload

[svenklemm]

The artifact upload for the downgrade test was using the filenames
of the update test and so would never upload any files.

[codecov]

Codecov Report

Merging #4041 (04bb3c4) into master (ae02934) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4041      +/-   ##
==========================================
- Coverage   90.70%   90.64%   -0.06%     
==========================================
  Files         214      214              
  Lines       38726    38733       +7     
==========================================
- Hits        35126    35110      -16     
- Misses       3600     3623      +23     

Impacted Files	Coverage Δ
src/chunk.h	100.00% <ø> (ø)
src/chunk.c	95.55% <100.00%> (+0.03%)	⬆️
src/planner.c	94.95% <100.00%> (+0.01%)	⬆️
src/ts_catalog/chunk_data_node.c	98.98% <100.00%> (ø)
tsl/src/fdw/modify_plan.c	90.16% <100.00%> (+0.16%)	⬆️
tsl/src/fdw/relinfo.c	96.22% <100.00%> (-0.10%)	⬇️
src/bgw/scheduler.c	83.04% <0.00%> (-3.51%)	⬇️
src/loader/bgw_message_queue.c	85.52% <0.00%> (-2.64%)	⬇️
src/loader/bgw_launcher.c	89.50% <0.00%> (-2.47%)	⬇️
tsl/src/reorder.c	84.79% <0.00%> (-0.30%)	⬇️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e5db6a9...04bb3c4. Read the comment docs.