[release-21.0] fix releasing the global read lock when mysqlshell backup fails (#17000)

[vitess-bot]

Description

This is a backport of #17000

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.16%. Comparing base (3fce8ca) to head (a3c19b9).
Report is 5 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/mysqlctl/fakemysqldaemon.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17012      +/-   ##
================================================
+ Coverage         67.13%   67.16%   +0.03%     
================================================
  Files              1570     1570              
  Lines            251255   251275      +20     
================================================
+ Hits             168671   168770      +99     
+ Misses            82584    82505      -79     

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