Download release assets for PowerShell PhpManager

[mlocati]

I just updated the PowerShell release script: now it builds an asset for every release.

Downloading such assets is preferable since:

• they contain updated metadata (useful for debugging)
• they contain only the production files
• they are pre-built: GitHub doesn't have to create a ZIP from a tag

[mlocati]

PS: having such assets is guaranteed by the release process of PhpManager.

Indeed, when I create a new release, I only have to push a new tag to the repository, where a script automatically does the following:

1. updates the module metadata
2. creates an asset containing the PowerShell module
3. publishes the new release to PowerShell Gallery
4. creates a GitHub release as draft, so that it's not counted when fetching api.github.com/repos/mlocati/powershell-phpmanager/releases/latest
5. attaches the asset to this draft release
6. marks the release as not draft (so that it's the latest one)