Use NormalizePath instead of Path.Combine for PackagedShimOutputDirectory

Follow-up to #49801 and #49288

We were still seeing an error during publish, note the duplicate slashes:

```
error NETSDK1152: Found multiple publish output files with the same relative path: /__w/1/s/artifacts/bin/dotnet-dump/Debug/shims/net8.0/win-x64/dotnet-dump.exe, /__w/1/s/artifacts/bin/dotnet-dump/Debug//shims/net8.0/win-x64/dotnet-dump.exe, /__w/1/s/artifacts/bin/dotnet-dump/Debug/shims/net8.0/win-x86/dotnet-dump.exe, /__w/1/s/artifacts/bin/dotnet-dump/Debug//shims/net8.0/win-x86/dotnet-dump.exe, /__w/1/s/artifacts/bin/dotnet-dump/Debug/shims/net8.0/osx-x64/dotnet-dump
```

Use the [MSBuild]::NormalizePath property function instead of Path.Combine to get a canonicalized path that doesn't contain duplicate slashes.

Author: akoeplinger

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.PackTool.targets

@@ -295,7 +295,7 @@ NOTE: This file is imported from the following contexts, so be aware when writin
       ApphostsForShimRuntimeIdentifiers="@(_ApphostsForShimRuntimeIdentifiers)"
       IntermediateAssembly="@(IntermediateAssembly->'%(FullPath)')"
       OutputType="$(OutputType)"
-      PackagedShimOutputDirectory="$([System.IO.Path]::Combine($(PackagedShimOutputRootDirectory), 'shims', $(_ToolPackShortTargetFrameworkName)))"
+      PackagedShimOutputDirectory="$([MSBuild]::NormalizePath($(PackagedShimOutputRootDirectory), 'shims', $(_ToolPackShortTargetFrameworkName)))"
       PackageId="$(PackageId)"
       PackageVersion="$(PackageVersion)"
       ShimRuntimeIdentifiers="@(_PackAsToolShimRuntimeIdentifiers)"
@@ -324,7 +324,7 @@ NOTE: This file is imported from the following contexts, so be aware when writin
 
   <Target Name="_ComputeExpectedEmbeddedApphostPaths">
     <GetEmbeddedApphostPaths
-      PackagedShimOutputDirectory="$([System.IO.Path]::Combine($(PackagedShimOutputRootDirectory), 'shims', $(_ToolPackShortTargetFrameworkName)))"
+      PackagedShimOutputDirectory="$([MSBuild]::NormalizePath($(PackagedShimOutputRootDirectory), 'shims', $(_ToolPackShortTargetFrameworkName)))"
       ShimRuntimeIdentifiers="@(_PackAsToolShimRuntimeIdentifiers)"
       ToolCommandName="$(ToolCommandName)">
 

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Publish.targets

@@ -1289,7 +1289,7 @@ Copyright (c) .NET Foundation. All rights reserved.
     </ItemGroup>
 
     <GetEmbeddedApphostPaths
-      PackagedShimOutputDirectory="$([System.IO.Path]::Combine($(PackagedShimOutputRootDirectory), 'shims', $(TargetFramework)))"
+      PackagedShimOutputDirectory="$([MSBuild]::NormalizePath($(PackagedShimOutputRootDirectory), 'shims', $(TargetFramework)))"
       ShimRuntimeIdentifiers="@(_PackAsToolShimRuntimeIdentifiers)"
       ToolCommandName="$(ToolCommandName)"
       >