arcade: darc is not a recognized cmdlet

• This issue is blocking
• This issue is causing unreasonable pain

For Validation-for-Promotion pipeline runs that have Arcade validate on branches of bellwether repos, the execution of the build-arcadewithrepo.ps1 script is failing at https://github.com/dotnet/arcade-validation/blob/master/eng/validation/build-arcadewithrepo.ps1#L233 for the Installer repo.

The logs show that darc was installed on the machine.

Example build failure: https://dnceng.visualstudio.com/internal/_build/results?buildId=748175&view=logs&j=6989144e-9c50-5586-7b61-a9af8cea5132&t=c78d63a3-c8c6-513a-1baf-fae22b0d7c09

Error:

D:\a\1\s\eng\validation\build-arcadewithrepo.ps1 : The term 'darc' is not recognized as the name of a cmdlet, 
function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the 
path is correct and try again.
At D:\a\_temp\dadf87a8-539d-4403-b166-9880f75e820d.ps1:3 char:1
+ eng\validation\build-arcadewithrepo.ps1 -azdoOrg dnceng -azdoProject  ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (darc:String) [build-arcadewithrepo.ps1], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException,build-arcadewithrepo.ps1