[workloads] Add locks for parallel NuGet access (dotnet#7813)

Android workload pack conversion has been failing intermittently:

    Generating package project: 'obj\Debug\src\msiPackage\x86\Microsoft.Android.Sdk.Windows\msi.csproj'
    convert.proj(72,5): error : One or more errors occurred.
    convert.proj(72,5): error : Failed to generate MSIs for workload packs.

    C:\Users\AzDevOps\.nuget\packages\wix\3.14.0-dotnet\tools\heat.exe dir obj\Debug\pkg\Microsoft.Android.Templates.31.0.100-rc.2.14 -cg CG_PackageContent -var var.SourceDir -ag -srd -sreg -dr InstallDir -o obj\Debug\src\msi\Microsoft.Android.Templates\31.0.100-rc.2.14\x64\PackageContent.wxs
    ##[error]light.exe(0,0): Error LGHT0001: The system cannot find the file specified. (Exception from HRESULT: 0x80070002)
    light.exe : error LGHT0001: The system cannot find the file specified. (Exception from HRESULT: 0x80070002) [C:\a\1\s\nuget-msi-convert\convert-v2\convert.proj]

      Exception Type: System.IO.FileNotFoundException

      Stack Trace:
         at Microsoft.Tools.WindowsInstallerXml.Cab.Interop.NativeMethods.CreateCabFinish(IntPtr contextHandle, IntPtr newCabNamesCallBackAddress)
         at Microsoft.Tools.WindowsInstallerXml.Cab.WixCreateCab.Complete(IntPtr newCabNamesCallBackAddress)
         at Microsoft.Tools.WindowsInstallerXml.CabinetBuilder.CreateCabinet(CabinetWorkItem cabinetWorkItem)
         at Microsoft.Tools.WindowsInstallerXml.CabinetBuilder.ProcessWorkItems()

I've added a lock to attempt to synchronize some .nupkg file access that
seemed to be problematic when executing asynchronously.

src/Microsoft.DotNet.Build.Tasks.Workloads/src/GenerateMsiBase.cs

@@ -110,6 +110,11 @@ public string SuppressIces
             set;
         }
 
+        /// <summary>
+        /// Used to synchronize access to the NuGet being extracted.
+        /// </summary>
+        private readonly object extractNuGetLock = new object();
+
         /// <summary>
         /// Generate a set of MSIs for the specified platforms using the specified NuGet package.
         /// </summary>
@@ -118,7 +123,11 @@ public string SuppressIces
         /// <param name="platforms"></param>
         protected IEnumerable<ITaskItem> Generate(string sourcePackage, string swixPackageId, string outputPath, WorkloadPackKind kind, params string[] platforms)
         {
-            NugetPackage nupkg = new(sourcePackage, Log);
+            NugetPackage nupkg = null;
+            lock (extractNuGetLock)
+            {
+                nupkg = new(sourcePackage, Log);
+            }
             List<TaskItem> msis = new();
 
             // MSI ProductName defaults to the package title and fallback to the package ID with a warning.
@@ -139,7 +148,10 @@ protected IEnumerable<ITaskItem> Generate(string sourcePackage, string swixPacka
             if ((kind != WorkloadPackKind.Library) && (kind != WorkloadPackKind.Template))
             {
                 Log.LogMessage(MessageImportance.Low, $"Extracting '{sourcePackage}' to '{packageContentsDirectory}'");
-                nupkg.Extract(packageContentsDirectory, exclusions);
+                lock (extractNuGetLock)
+                {
+                    nupkg.Extract(packageContentsDirectory, exclusions);
+                }
             }
             else
             {
@@ -188,9 +200,12 @@ protected IEnumerable<ITaskItem> Generate(string sourcePackage, string swixPacka
                     SourceDirectory = packageContentsDirectory
                 };
 
-                if (!heat.Execute())
+                lock (extractNuGetLock)
                 {
-                    throw new Exception($"Failed to harvest package contents.");
+                    if (!heat.Execute())
+                    {
+                        throw new Exception($"Failed to harvest package contents.");
+                    }
                 }
 
                 // Compile the MSI sources