Fix ineffective assignment in db/mock/template.go

[detiber]

Description

While updating and rebasing #266, I noticed that golangci-lint was finding a new issue in our db mocking library. This PR addresses that issue

How Has This Been Tested?

I verified tests continued to work after updating the receiver type of the methods that modified the mock db variable.

How are existing users impacted? What migration steps/scripts do we need?

This change only impacts tests and no migrations are needed.

[codecov]

Codecov Report

Merging #511 (fbe7262) into master (7f829f5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #511   +/-   ##
=======================================
  Coverage   32.70%   32.70%           
=======================================
  Files          44       44           
  Lines        3137     3137           
=======================================
  Hits         1026     1026           
  Misses       2019     2019           
  Partials       92       92           

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