fix: don't use Command::new(bin_name) as it won't work on Windows

[SteveLauC]

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• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
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What does this PR do

On Windows, involving a command using Command::new("binary_name") can fail with error Command not found even though the corresponding binary is installed and put somewhere under $PATH, for example, #720.

There is a Rust issue for it: rust-lang/rust#37519

A workaround is to use the which crate, which works in a way closer to how PowerShell handles cmd resolution, Topgrade's helper function require() uses which() under the hood, so this PR replaces those Command::new("binary-name-string") with Command::new(a_path_returned_from_which).

Closes #720

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 5.03%. Comparing base (f4e0258) to head (d0e8b7e).

Files	Patch %	Lines
src/steps/generic.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #750   +/-   ##
=====================================
  Coverage   5.03%   5.03%           
=====================================
  Files         37      37           
  Lines      12085   12085           
=====================================
  Hits         609     609           
  Misses     11476   11476           

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