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

[MatejKafka]

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]

Please use a branch other than develop and update the changlog.

[MatejKafka]

Please use a branch other than develop and update the changlog.

Added a changelog entry. What branch should I use? The PR guidelines explicitly say to target develop.

[niheaven]

I mean your local branch. You should create a new branch and make PR from that branch.

[MatejKafka]

Whatever. #5998