Remove protoc conditional properties (#4817)

* Remove protoc conditional properties

* Fix duplicate brace

csharp/Google.Protobuf.Tools.targets

@@ -1,16 +1,11 @@
-<Project>
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
     <PropertyGroup>
-        <protoc_linux64>$(MSBuildThisFileDirectory)..\tools\linux_x64\protoc</protoc_linux64>
-        <protoc_linux86>$(MSBuildThisFileDirectory)..\tools\linux_x86\protoc</protoc_linux86>
-        <protoc_macosx64>$(MSBuildThisFileDirectory)..\tools\macosx_x64\protoc</protoc_macosx64>
-        <protoc_macosx86>$(MSBuildThisFileDirectory)..\tools\macosx_x86\protoc</protoc_macosx86>
-        <protoc_windows64>$(MSBuildThisFileDirectory)..\tools\windows_x64\protoc.exe</protoc_windows64>
-        <protoc_windows86>$(MSBuildThisFileDirectory)..\tools\windows_x86\protoc.exe</protoc_windows86>
-        <protoc Condition="'$([MSBuild]::IsOsPlatform(Linux))' And '$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture)'=='X64'">$(protoc_linux64)</protoc>
-        <protoc Condition="'$([MSBuild]::IsOsPlatform(Linux))' And '$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture)'=='X86'">$(protoc_linux86)</protoc>
-        <protoc Condition="'$([MSBuild]::IsOsPlatform(OSX))' And '$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture)'=='X64'">$(protoc_macosx64)</protoc>
-        <protoc Condition="'$([MSBuild]::IsOsPlatform(OSX))' And '$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture)'=='X86'">$(protoc_macosx86)</protoc>
-        <protoc Condition="'$([MSBuild]::IsOsPlatform(Windows))' And '$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture)'=='X64'">$(protoc_windows64)</protoc>
-        <protoc Condition="'$([MSBuild]::IsOsPlatform(Windows))' And '$([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture)'=='X86'">$(protoc_windows86)</protoc>
+        <protoc_tools>$([System.IO.Path]::GetFullPath('$(MSBuildThisFileDirectory)/../tools/'))</protoc_tools>
+        <protoc_linux64>$([System.IO.Path]::GetFullPath('$(protoc_tools)/linux_x64/protoc'))</protoc_linux64>
+        <protoc_linux86>$([System.IO.Path]::GetFullPath('$(protoc_tools)/linux_x86/protoc'))</protoc_linux86>
+        <protoc_macosx64>$([System.IO.Path]::GetFullPath('$(protoc_tools)/macosx_x64/protoc'))</protoc_macosx64>
+        <protoc_macosx86>$([System.IO.Path]::GetFullPath('$(protoc_tools)/macosx_x86/protoc'))</protoc_macosx86>
+        <protoc_windows64>$([System.IO.Path]::GetFullPath('$(protoc_tools)/windows_x64/protoc.exe'))</protoc_windows64>
+        <protoc_windows86>$([System.IO.Path]::GetFullPath('$(protoc_tools)/windows_x86/protoc.exe'))</protoc_windows86>
     </PropertyGroup>
-</Project>
+</Project>