Fix warning 'Executable bit is not set on target script, sourcing instead of executing.'

[wtgodbe]

Part of #22240

Happens during 'Setup Private Feed Credentials' on non-windows builds. @dougbu claims this will be fixed by the next Arcade update.

Full warning text:

##[warning]Executable bit is not set on target script, sourcing instead of executing. More info at https://github.com/Microsoft/azure-pipelines-tasks/blob/master/docs/bashnote.md

[dougbu]

I actually claimed dotnet/arcade#4894 should have fixed it already. @riarenas haven't we picked up that change in this repo? If we have, why isn't the exec bit set correctly on eng/common/SetupNugetSources.sh in the CI?

[riarenas]

That change flowed a long time ago, but I think there's a problem with that particular file right now: dotnet/arcade-services#2525

[wtgodbe]

Resolved by #22521