MSBuild command line failing on first run with "FixXFCoreAssembly" -- FAILED.

[PureWeen]

Description

Running msbuild from the command line fails the first time

Steps to Reproduce

1. git clean -xdf
2. msbuild Xamarin.Forms.sln /t:restore
3. msbuild Xamarin.Forms.sln /p:CreateAllAndroidTargets=true

Using "FixXFCoreAssembly" task from assembly "D:\Development\Xamarin.Forms\3.5\Xamarin.Forms.Core\..\XFCorePostProcessor.Tasks\bin\Debug\net461\XFCorePostProcessor.Tasks.dll".
Task "FixXFCoreAssembly"
  Generating backcompat code for #2835
    already executed
Done executing task "FixXFCoreAssembly" -- FAILED.
Done building target "XFCorePostProcessor" in project "Xamarin.Forms.Core.csproj" -- FAILED.

If you run msbuild Xamarin.Forms.sln /p:CreateAllAndroidTargets=true again then it works

Expected Behavior

Should only have to run msbuild once

Actual Behavior

Have to run it twice because the first time fails

Basic Information

• Version with issue: 3.5.0
• Last known good version: earlier version of 3.5.0

[StephaneDelcroix]

The modified task return failure if it was already executed, maybe it should not.
Some remarks:

• what's going on here so that task is executed twice ?
• I changed the .csproj on master to generate a stamp file, so this shouldn't happen ([build] add a stamp file to shamehat #4591)