fix indexer reindex bug when gitea restart

[lunny]

Caused by #3452 and will fix #5325.
On #3452, we add an index version checking on metajson file, but forgot to save it on the file when create a new indexer directory. :(

[codecov-io]

Codecov Report

Merging #5563 into master will decrease coverage by <.01%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master    #5563      +/-   ##
==========================================
- Coverage   37.57%   37.57%   -0.01%     
==========================================
  Files         321      321              
  Lines       47182    47192      +10     
==========================================
+ Hits        17729    17732       +3     
- Misses      26913    26918       +5     
- Partials     2540     2542       +2

Impacted Files	Coverage Δ
modules/indexer/issue.go	65.82% <55.55%> (-1.75%)	⬇️
modules/indexer/repo.go	63.35% <55.55%> (-0.93%)	⬇️
models/repo_list.go	63.29% <0%> (-1.27%)	⬇️
models/repo_indexer.go	48.3% <0%> (+0.42%)	⬆️

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