fix(build): restore the tools once per build instead of once per project

A solution restores its projects in parallel, so `ToolRestore` ran `dotnet tool restore` in every project
at the same time and all of them touched the same marker file. The `Restore dependencies` step of
the Copilot setup, which restores the solution, failed on Linux after `ToolRestore` started to run
on every restore.

Every project now delegates to one instance of `Directory.Build.targets`, which MSBuild builds once per build,
so the tools are restored once and the other projects wait for that run.
The Copilot setup no longer restores the tools on its own before restoring the solution.

`Format` says why it runs Fantomas twice.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: xperiandri

.github/workflows/copilot-setup-steps.yml

@@ -37,6 +37,5 @@ jobs:
             10.x
 
       - name: Restore dependencies
-        run: |
-          dotnet tool restore
-          dotnet restore FSharp.Collections.Immutable.slnx
+        # Restoring the solution restores the local tools first (see Directory.Build.targets)
+        run: dotnet restore FSharp.Collections.Immutable.slnx

Directory.Build.targets

@@ -11,8 +11,18 @@ See https://learn.microsoft.com/en-us/visualstudio/msbuild/customize-by-director
     <_DotnetFantomasOutputFile>$(BaseIntermediateOutputPath)dotnet-fantomas-msbuild-$(NETCoreSdkVersion)-$(OS)</_DotnetFantomasOutputFile>
   </PropertyGroup>
 
-  <!-- Make sure that dotnet tools are restored before restoring any project -->
-  <Target Name="ToolRestore" BeforeTargets="Restore;CollectPackageReferences" Inputs="$(_DotnetToolManifestFile)" Outputs="$(_DotnetToolRestoreOutputFile)">
+  <!--
+    Make sure that dotnet tools are restored before restoring any project.
+    A solution restores its projects in parallel, so restoring the tools in each of them ran `dotnet tool restore`
+    concurrently and raced on the marker file.
+    Every project therefore delegates to one instance of this file: MSBuild runs a target once per project instance,
+    so the tools are restored once per build, and the projects that ask at the same time wait for that one run.
+  -->
+  <Target Name="ToolRestore" BeforeTargets="Restore;CollectPackageReferences">
+    <MSBuild Projects="$(MSBuildThisFileFullPath)" Targets="ToolRestoreOnce" Properties="_DotnetToolRestoreOutputFile=$(_DotnetToolRestoreOutputFile)" RemoveProperties="TargetFramework;RuntimeIdentifier" />
+  </Target>
+
+  <Target Name="ToolRestoreOnce" Inputs="$(_DotnetToolManifestFile)" Outputs="$(_DotnetToolRestoreOutputFile)">
     <Exec Command="dotnet tool restore" WorkingDirectory="$(MSBuildThisFileDirectory)" StandardOutputImportance="High" StandardErrorImportance="High" />
     <MakeDir Directories="$(_BuildProjBaseIntermediateOutputPath)"/>
     <Touch Files="$(_DotnetToolRestoreOutputFile)" AlwaysCreate="True" ForceTouch="True" />
@@ -51,6 +61,7 @@ See https://learn.microsoft.com/en-us/visualstudio/msbuild/customize-by-director
     <MSBuild Projects="$(MSBuildProjectFullPath)" Targets="Format" Properties="_FantomasFormatInstance=true" RemoveProperties="TargetFramework;RuntimeIdentifier" />
   </Target>
 
+  <!-- Fantomas runs twice on purpose: one pass is not always idempotent, and the second one settles what the first moved -->
   <Target Name="Format" Inputs="@(Compile)" Outputs="$(_DotnetFantomasOutputFile)">
     <Exec Command="dotnet fantomas $(MSBuildProjectDirectory)" StandardOutputImportance="High" StandardErrorImportance="High" WorkingDirectory="$(MSBuildThisFileDirectory)" ContinueOnError="WarnAndContinue" />
     <Exec Command="dotnet fantomas $(MSBuildProjectDirectory)" StandardOutputImportance="High" StandardErrorImportance="High" WorkingDirectory="$(MSBuildThisFileDirectory)" ContinueOnError="WarnAndContinue" />