[enhancement]: Allow using nuspec via DotNetCoreCLI@2

[alex-ritter]

Task name

No response

Describe your feature request here

Currently, as it stands we cannot use DotNetCoreCLI@2 to create nuget packages based off of a provided nuspec file with a dll.

When using DotNetCoreCLI@2 as below.

• task: DotNetCoreCLI@2
  displayName: 'Pack Nuget Files'
  condition: and(
  ne(variables['Build.Reason'], 'IndividualCI'),
  ne(variables['Build.Reason'], 'PullRequest')
  )
  inputs:
  command: pack
  packagesToPack: '**/*.nuspec'
  nobuild: true
  packDestination: '$(Build.ArtifactStagingDirectory)/publish/release-files/packages'
  versioningScheme: 'byEnvVar'
  versionEnvVar: 'PackageVersion'
  workingDirectory: '$(Build.ArtifactStagingDirectory)/publish/release-files'

We are getting an error

'error MSB1003: Specify a project or solution file. The current working directory does not contain a project or solution file.'

There are nuget files specified in the '$(Build.ArtifactStagingDirectory)/publish/release-files' and manually specifying the nuspec file by name does not work.

If we try to directly use the nuget command, we require mono to be installed and in other issues, it is advised to be using DotNetCoreCLI, but due to our processes we need to be directly making nuget packages from a nuspec file, and that is currently not possible with DotNetCoreCLI.