Bump nuget.exe to 6.6.2

[libs][TimeZoneInfo] Bound transition time conversion (#21686)

Co-authored-by: mdh1418 <mitchhwang1418@gmail.com>

[Mono.Security] Add additional validation to AuthenticodeDeformatter

Check that the Extended Key Usage extension in the signing certificate contains the OID for code signing so that we reject other types of certificates.

Bump nuget.exe to v6.6.1

Fix xar url again

(cherry picked from commit 3005442)

Change download URL for PCL reference assemblies

Bump NuGetSdkResolver in msbuild

Backport fixes for sharing wrappers when type attributes are involved (…

…#21537)

* [wasm] Fix the handling of i8/u8 in get_wrapper_shared_type_full (). (#19859)

Previously, these were returned verbatim, which caused sharing issues when the type had
attributes.

Fixes #19841.

* [aot] Fix the handling of r4/r8 parameter types with attributes during generic sharing. (#20217)

The attributes need to be ignored as with the other types, otherwise gsharedvt wrappers for signatures with
parameters like double f = default will not be found.

Fixes #20195.

Co-authored-by: Zoltan Varga <vargaz@gmail.com>

Backporting dotnet/runtime#59861 (#21532)

Fixes dotnet/runtime#72181

Co-authored-by: Ivan Povazan <ivan.povazan@gmail.com>

Backport dotnet/runtime#71436 (#21519)

Backport dotnet/runtime#71436
Backport of #21516 to 2020-02

Co-authored-by: Thays Grazia <thaystg@gmail.com>