Fix SkipScan crash due to pruned unique path

[erimatnor]

When SkipScan paths are added it can result in a crash if the new
SkipScan dominates the PostgreSQL planned unique path. In that case,
the previous unique path is pruned and freed, leading to a potential
memory corruption since a pointer to the original unique path is used
when looping over all subpaths to try to apply SkipScan. Thus, if the
original unique path is pruned, and there are multiple subpaths, a
crash will occur in the next iteration over the subpaths.

The crash is fixed by making a shallow copy of the original unique
path while iterating the subpaths.

[erimatnor]

I've tried to add a test case but it is very difficult to create on for the regression suite. Basically, these conditions must be fulfilled:

• There must be multiple paths considered for scanning the base relation
• The SkipScan path must dominate a previous unique path so that it is pruned
• The pruned path cannot be an indexpath (they're never pfreed in add_path)

[codecov]

Codecov Report

Merging #3881 (095991a) into master (8afac60) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3881   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files         213      213           
  Lines       37966    37967    +1     
=======================================
+ Hits        34390    34391    +1     
  Misses       3576     3576           

Impacted Files	Coverage Δ
tsl/src/nodes/skip_scan/planner.c	96.21% <100.00%> (+0.02%)	⬆️
tsl/src/bgw_policy/job.c	88.04% <0.00%> (-0.05%)	⬇️

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