Small fix for network interruption in certification flow

[berndverst]

Description

The defer method's proper channel closing for the timer was causing a hang when a timer had correctly completed. This change ensures there will be no hanging.

Issue reference

#1137

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly

[daixiang0]

Do you consider abort defer and using channels to control shutdown cases like this?

[codecov]

Codecov Report

Merging #1237 (f1c0139) into master (426c458) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1237      +/-   ##
==========================================
- Coverage   35.07%   35.04%   -0.04%     
==========================================
  Files         147      147              
  Lines       12747    12747              
==========================================
- Hits         4471     4467       -4     
- Misses       7795     7801       +6     
+ Partials      481      479       -2     

Impacted Files	Coverage Δ
state/cloudstate/cloudstate_crdt.go	8.67% <0.00%> (-1.66%)	⬇️

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