chore: Add a env prefix

[kukugi]

Description

add env prefix for setting viper value useful

related: https://github.com/line/lbm/issues/1269

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write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #200 (8a081c6) into v2/develop (23a0f9e) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           v2/develop     #200   +/-   ##
===========================================
  Coverage       53.45%   53.45%           
===========================================
  Files             653      653           
  Lines           47289    47289           
===========================================
  Hits            25280    25280           
  Misses          19157    19157           
  Partials         2852     2852