FIX - Changed all the s3 key string formatting to use path

[BigMakak]

Changed all the s3 key string formatting to use path.join instead of filepath, to remove unnecessary OS prefixes

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.68%. Comparing base (49206e9) to head (602b87c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
s3sync.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
+ Coverage   81.25%   81.68%   +0.43%     
==========================================
  Files           5        5              
  Lines         416      415       -1     
==========================================
+ Hits          338      339       +1     
+ Misses         47       46       -1     
+ Partials       31       30       -1     

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