clap-utils: Fix signer resolution on Windows (backport #17343)

[mergify]

This is an automatic backport of pull request #17343 done by Mergify.
Cherry-pick of e320af9 has failed:

On branch mergify/bp/v1.6/pr-17343
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	both modified:   clap-utils/src/keypair.rs

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[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #17371 (4d409a2) into v1.6 (3f6964d) will increase coverage by 0.0%.
The diff coverage is 85.7%.

@@           Coverage Diff           @@
##             v1.6   #17371   +/-   ##
=======================================
  Coverage    82.6%    82.6%           
=======================================
  Files         423      423           
  Lines      116118   116117    -1     
=======================================
+ Hits        95958    95970   +12     
+ Misses      20160    20147   -13