fix docfx.console nuget package

[tibel]

fixes #7557

[jamiehankins]

I'm guessing that when target was removed in #694, DocFX started relying on NuGet to keep the directory name if target is empty. NuGet 5.10.0 seems to have changed that behavior.

This seems like a better fix that pinning to an old version of NuGet.

[yufeih]

@jamiehankins @tibel , I'm preferring #7571 over this PR because this syntax <file src="build/**" target="build"/> can have different interpretations:

1. If src is treated as a transparent glob pattern, then the glob matcher for build/a.txt yields build/a.txt and place it at build/build/a.txt like the behavior in 5.9.1
2. If NuGet tries to remove common prefix using the target attribute, then the glob matcher for build/a.txt yields build/a.txt and then turn it into a.txt before placing it to build/a.txt, like the behavior in 5.10.0
3. If NuGet tries to be really clever and look for ** in the glob pattern, then the glob matcher for build/a.txt yields a.txt place it at build/a.txt, like the behavior in 5.10.0. (But what does it mean for glob patterns like build/**/a** or buil?/**?)

I'd like to wait for the NuGet team to clarify the behavior on NuGet/Home#11125, maybe it is a regression, maybe they intentionally adjust the behavior for some reason.