Windows friendly stdin reads for passwords

[SuburbanDad]

TODO(you): choose "part of" or "resolves" and the associated github issue #.

[Part of|Resolves] #5008

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Description

On windows, syscall.Stdin is not an int. This PR changes reads of stdin (currently only keystore passwords) to cast stdin filehandle to int explicitly. This fixes the windows validator build in #4945

Smoke tested for regression with amd64 build

Write why you are making the changes in this pull request

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Link anything that would be helpful or relevant to the reviewers

Target //validator:validator up-to-date:
  bazel-bin/validator/windows_amd64_stripped/validator.exe
INFO: Elapsed time: 173.822s, Critical Path: 59.89s
INFO: 754 processes: 754 processwrapper-sandbox.
INFO: Build completed successfully, 783 total actions

[shayzluf]

plese run goimports -w .

[codecov]

Codecov Report

Merging #5010 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5010   +/-   ##
=======================================
  Coverage   43.10%   43.10%           
=======================================
  Files         264      264           
  Lines       20331    20331           
=======================================
  Hits         8763     8763           
  Misses      10052    10052           
  Partials     1516     1516           

[shayzluf]

lgtm