Set apphost and bundle file permission to 755₈

When building a .net core 3 app, the SDK currently simply copies the apphost template (including its permissions) from the install-location. 

This caused two problems in Unix systems:
* If the dotnet install location is write-protected, the build fails when SDK tries update the apphost (to set the app-path, etc.)  (dotnet/core-setup#8511)
* The built apphost can only be run by the owner (dotnet/core-setup#7062)

This change explicitly sets the file permissions of the Apphost in the SDK to fix the above issues.




Commit migrated from https://github.com/dotnet/core-setup/commit/a6a9202dc186f622f5df7125b12d95d9107c193a