Building .sln files within a ??? directory fails on macOS

### Steps to reproduce

1. Make a `.sln` of any kind, put it in a directory named `???` on macOS.

2. `cd` into the directory, and build with either:

```
$ cd \?\?\?
$ msbuild foo.sln
$ dotnet build foo.sln
```
3. fails with:

```
/Library/Frameworks/Mono.framework/Versions/6.12.0/lib/mono/xbuild/Current/Microsoft.Common.props(62,3):
error MSB4019: The imported project "/Users/jopepper/Projects/???/foo/obj/foo.csproj.*.props" was not found.
Confirm that the expression in the Import declaration "/Users/jopepper/Projects/%3f%3f%3f/foo/obj/foo.csproj.*.props" is correct, and that the file exists on disk.
```

Sample (binlogs inside): [???.zip](https://github.com/microsoft/msbuild/files/4736412/default.zip)

### Expected behavior

Things work inside a directory with question marks.

### Actual behavior

It appears that question marks break a `.sln` build.

### Environment data

Since this fails under Mono & .NET Core, it probably happens on any MSBuild running on macOS.

`msbuild /version` output:
```
Microsoft (R) Build Engine version 16.6.0-ci for Mono
Copyright (C) Microsoft Corporation. All rights reserved.
16.6.0.16201
...
Microsoft (R) Build Engine version 16.7.0-preview-20270-03+bee129d1b for .NET
Copyright (C) Microsoft Corporation. All rights reserved.
16.7.0.27003
```

OS info:

macOS Catalina 10.15.4

Reproduce the failure on macOS using the commands in the issue and a solution under a directory named `???`. Inspect the supplied binlogs and the import reported at `Microsoft.Common.props(62,3)` to trace how the path is transformed. Done means both `msbuild foo.sln` and `dotnet build foo.sln` succeed from that directory.

Written by the indexing model from the issue text.