Fix charset was not saved after installation finished

[lunny]

On #6992, we introduced CHARSET on database seciton to app.ini. But unfortunately, I missed to save CHARSET to app.ini after installation finished. This PR will fix that.

[codecov-io]

Codecov Report

Merging #7048 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #7048      +/-   ##
==========================================
+ Coverage   41.45%   41.45%   +<.01%     
==========================================
  Files         442      442              
  Lines       59608    59610       +2     
==========================================
+ Hits        24708    24709       +1     
- Misses      31670    31672       +2     
+ Partials     3230     3229       -1

Impacted Files	Coverage Δ
routers/install.go	0% <0%> (ø)	⬆️
models/repo_list.go	72.08% <0%> (-1.02%)	⬇️
models/gpg_key.go	56.66% <0%> (+0.83%)	⬆️

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