fix(core): Search for Git executable instead of any cmdlet

[MatejKafka]

#5979

Motivation and Context

Before this commit, if a user had a custom alias or function for git specified in their PowerShell profile, Get-HelperPath would return path to the PowerShell module where the function is defined instead of path to the actual Git binary.

How Has This Been Tested?

Tested interactively, the code seems reasonably obvious to me. To test the fix, run the following snippet:

New-Module -Script {function git {}}
scoop update

Without this PR, the invocation results in the folowing error:

> scoop update
&: D:\_custom\scoop\app\apps\scoop\current\lib\core.ps1:273
Line |
 273 |          return & $git @ArgumentList
     |                   ~~~~
     | The term '__DynamicModule_cbe178d9-14c2-494d-8004-fbdc292d59d2' is not recognized as a name of a cmdlet, function, script file, or executable program. Check the spelling of the name, or if a path was
     | included, verify that the path is correct and try again.

Checklist:

• I have read the Contributing Guide.
• I have ensured that I am targeting the develop branch.
• [-] I have updated the documentation accordingly.
• [-] I have updated the tests accordingly.
• I have added an entry in the CHANGELOG.

[niheaven]

Hmm, would you please sign your commit?

[MatejKafka]

Nope. If you want the commit, merge it (or just make the change in your own commit, I don't care about attribution), can't be bothered to setup a signature.