Skip unmanaged dlls in MobileILStrip task

[jkurdek]

Fixes dotnet/runtime#101967
Follow up to the runtime change dotnet/runtime#106267

[github-actions]

⚠️ Your code has been reformatted. ⚠️

If this is not desired, add the actions-disable-autoformat label, and revert the reformatting commit.

If files unrelated to your change were modified, try reverting the reformatting commit + merging with the target branch (and push those changes).

[rolfbjarne]

@jkurdek unfortunately we can't currently build PRs from forks, so can you push your branch to origin and recreate the PR?