fix code cov

[BennyThink]

worth mentioning:

1. this commit remove on push d104c1f
2. this commit make clean will delete more files, including codecov.txt 23bbed8
3. although on push was added back here 13281d9 , we would first run make test and then make

So codecov.txt is missing! So we're missing code coverage data for more than a year!

[github-actions]

ghcr.io/webp-sh/webp_server_go (debian 12.0)
============================================
Total: 0 (HIGH: 0, CRITICAL: 0)

[codecov]

Codecov Report

Merging #248 (9253631) into master (9efead2) will decrease coverage by 54.53%.
The diff coverage is 29.05%.

@@             Coverage Diff             @@
##           master     #248       +/-   ##
===========================================
- Coverage   83.57%   29.05%   -54.53%     
===========================================
  Files           6        2        -4     
  Lines         542      179      -363     
===========================================
- Hits          453       52      -401     
- Misses         61      122       +61     
+ Partials       28        5       -23     

Impacted Files	Coverage Δ
helper/helper.go	20.00% <20.00%> (ø)
config/config.go	87.50% <87.50%> (ø)

... and 2 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more