fix: Update subpath from DeepCopy instead of pointer

[odinnordico]

Pull request

What this PR does / why we need it

When doing the Merge over WorkloadSpec, on each "sub-Merge", ResetSource (which converts Source = nil) was used but keeping a stash with the previous value of the section (like Source) but as those are pointer references then when ResetSource is called it changes the whole reference, then the validations in others struct members fail.

This PR uses DeepCopy to keep the "stash" so the further validations/checks are against the actual values.

Which issue(s) this PR fixes

Fixes #565

Describe testing done for PR

• Create a workload from a local source

• Create a file to update the workload

• Update the workload from a file, changing the subpath

Additional information or special notes for your reviewer

[codecov-commenter]

Codecov Report

Merging #580 (181248e) into main (fd35b06) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 181248e differs from pull request most recent head 6668c91. Consider uploading reports for the commit 6668c91 to get more accurate results

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
+ Coverage   85.76%   85.78%   +0.02%     
==========================================
  Files          68       68              
  Lines        4249     4255       +6     
==========================================
+ Hits         3644     3650       +6     
  Misses        473      473              
  Partials      132      132              

Impacted Files	Coverage Δ
pkg/apis/cartographer/v1alpha1/workload_helpers.go	94.87% <100.00%> (+0.07%)	⬆️

[warango4]

LGTM