Builds are failing on Ubuntu when attempting to restore packages from NuGet.org

[gep13]

When building on Mono, we see the following output when attempting to restore packages from NuGet.org:

                 [exec] Restoring NuGet package NUnit.Runners.2.6.4.
                 [exec] WARNING: Unable to find version '2.6.4' of package 'NUnit.Runners'.
                 [exec]   https://api.nuget.org/v3/index.json: Unable to load the service index for source https://api.nuget.org/v3/index.json.
                 [exec]   The SSL connection could not be established, see inner exception.
                 [exec]   Authentication failed, see inner exception.
                 [exec]   Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED
                 [exec]     at /build/mono-6.12.0.122/external/boringssl/ssl/handshake_client.c:1132
                 [exec]   https://www.nuget.org/api/v2/: Unable to load the service index for source https://www.nuget.org/api/v2/.
                 [exec]   The SSL connection could not be established, see inner exception.
                 [exec]   Authentication failed, see inner exception.
                 [exec]   Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED
                 [exec]     at /build/mono-6.12.0.122/external/boringssl/ssl/handshake_client.c:1132

According to this report:

mono/mono#12406 (comment)

This is a known problem when restoring packages on Mono, although this is the first time that we are seeing this.

The workaround is to append the -DisableParallelProcessing argument when invoking NuGet.exe. This can be done within the .uppercut file.