Fix #154 CIでBotanのビルドがうまくいかない

[shoo]

Botanが再度使用可能になったため、過去に削除したコードを復活させます。

[codecov-commenter]

Codecov Report

Merging #201 (1e07cc1) into master (816d33b) will increase coverage by 0.11%. The diff coverage is 100.00%.

:exclamation: Current head 1e07cc1 differs from pull request most recent head 987db08. Consider uploading reports for the commit 987db08 to get more accurate results

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@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
+ Coverage   97.41%   97.52%   +0.11%     
==========================================
  Files          41       42       +1     
  Lines        2318     2428     +110     
==========================================
+ Hits         2258     2368     +110     
  Misses         60       60              

Files Changed	Coverage Δ
thirdparty/botan/source/botan_usage/example.d	100.00% <100.00%> (ø)

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