[release-23.0] mysqlshellbackupengine: use fileutil.SafePathJoin(...) to build path (#19484)

[vitess-bot]

Description

This is a backport of #19484

[codecov]

Codecov Report

❌ Patch coverage is 44.00000% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.45%. Comparing base (10d1e79) to head (69ec791).
⚠️ Report is 3 commits behind head on release-23.0.

Files with missing lines	Patch %	Lines
go/vt/mysqlctl/mysqlshellbackupengine.go	44.00%	14 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           release-23.0   #19491   +/-   ##
=============================================
  Coverage         69.45%   69.45%           
=============================================
  Files              1606     1606           
  Lines            214903   214956   +53     
=============================================
+ Hits             149262   149303   +41     
- Misses            65641    65653   +12     

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