dotnetup: Handle all unversioned files in dotnet root during installation (not just the muxer)

[nagilson]

Context

In PR #52696, dotnetup gained logic to safely handle the muxer (dotnet/dotnet.exe) during archive extraction — comparing runtime versions to decide whether to replace it and gracefully handling in-use scenarios.

However, the dotnet root contains other unversioned files beyond the muxer that are not currently handled by this logic. These include:

• The dnx script
• License files (e.g., LICENSE.txt, ThirdPartyNotices.txt)

Proposal

Extend MuxerHandler (or introduce a broader handler) to manage all unversioned files stored in the dotnet root as a group. The rule should be:

• If we determine the muxer should be replaced (because a newer runtime was installed), replace all unversioned root files from the archive as well.
• If the muxer should not be replaced, skip all unversioned root files.
• Apply the same in-use / error handling strategy (warn or fail based on --require-muxer-update) to these files.

This keeps the dotnet root files consistent and avoids a situation where a new muxer is paired with stale license/script files or vice versa.

References

• PR #52696 review comment (dsplaisted) — "I think we will want to handle not only the muxer but also any unversioned files stored in the dotnet root (which right now includes the dnx script and license files). I think what we want to do is handle all of those files as a group, ie if we replace the muxer then we replace the other files too."