OutputPath Error Check Fails in EndToEnd.csproj

### Issue Description
We recently [added BaseOutputPath to common targets](https://github.com/dotnet/msbuild/pull/5238) which added error checks in Microsoft.Common.CurrentVersion.targets like so:

```xml

```

These errors were actually thrown in DDRITs test project `EndToEnd.csproj` here: https://devdiv.visualstudio.com/DevDiv/_git/VS/pullrequest/292082
```
[04:05:54.601] Failed build step "msbuild ..\EndToEnd.csproj" with exit code 1
--
[04:05:54.601] Failed build step "msbuild ..\EndToEnd.csproj" with exit code 1
[04:05:54.601]
[04:05:54.601] Microsoft (R) Build Engine version 16.9.0-preview-20607-04+1ff34e830 for .NET Framework
[04:05:54.601] Copyright (C) Microsoft Corporation. All rights reserved.
[04:05:54.601] Build started 12/9/2020 4:05:54 AM.
[04:05:54.601] Project "C:\Test\Containers\VC.Tests.IDE\Tests\Integration\DDRITs\EndToEnd.csproj" on node 1 (default targets).
[04:05:54.601] C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(824,5): error : The OutputPath must end with a trailing slash. [C:\Test\Containers\VC.Tests.IDE\Tests\Integration\DDRITs\EndToEnd.csproj]
[04:05:54.601] Done Building Project "C:\Test\Containers\VC.Tests.IDE\Tests\Integration\DDRITs\EndToEnd.csproj" (default targets) -- FAILED.
[04:05:54.601] Build FAILED.
[04:05:54.601] "C:\Test\Containers\VC.Tests.IDE\Tests\Integration\DDRITs\EndToEnd.csproj" (default target) (1) ->
[04:05:54.601] (_CheckForInvalidConfigurationAndPlatform target) ->
[04:05:54.601] C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\Current\Bin\Microsoft.Common.CurrentVersion.targets(824,5): error : The OutputPath must end with a trailing slash. [C:\Test\Containers\VC.Tests.IDE\Tests\Integration\DDRITs\EndToEnd.csproj]
[04:05:54.601] 0 Warning(s)
[04:05:54.601] 1 Error(s)
[04:05:54.601] Time Elapsed 00:00:00.18
[04:05:54.601]
```

To Do:
- [ ] Download the `EndToEnd.csproj` project from the VS repo.
- [ ] Repro the failure.
- [ ] Figure out why `EndToEnd.csproj` has `OutputPath` set to some value that doesn't end in a slash when it hits that target.