style: prefer formatting prints to mock output instead of writing string

[zimeg]

Summary

This PR fixes a formatting issue on main to avoid build errors after updates to golangci-lint tooling:

golangci-lint run
internal/config/experiments_test.go:303:26: printf: missing ... in args forwarded to printf-like function (govet)
                mockOutput.WriteString(fmt.Sprintf(format, a))
                                       ^
1 issues:
* govet: 1
make: *** [Makefile:55: lint] Error 1

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.61%. Comparing base (f30df06) to head (f8f388f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #250      +/-   ##
==========================================
+ Coverage   63.07%   64.61%   +1.53%     
==========================================
  Files         212      212              
  Lines       21857    17505    -4352     
==========================================
- Hits        13786    11310    -2476     
+ Misses       7008     5130    -1878     
- Partials     1063     1065       +2     

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[zimeg]

🧪 note: Merging this to unblock tests on main for upcoming releases!