Add retry to symlink test

[danmoseley]

Fix#72902

[ghost]

Tagging subscribers to this area: @dotnet/area-extensions-filesystem
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Fix #72902

Author:	danmoseley
Assignees:	-
Labels:	area-Extensions-FileSystem
Milestone:	-

[danmoseley]

@Dotnet/dnceng do we have appropriate retries here?

2022-07-29T04:04:17.4016275Z Failed to download package 'NuGet.ProjectModel.6.2.1' from 'https://pkgs.dev.azure.com/dnceng/9ee6d478-d288-47f7-aacc-f6e6d082ae6d/_packaging/45bacae2-5efb-47c8-91e5-8ec20c22b4f8/nuget/v3/flat2/nuget.projectmodel/6.2.1/nuget.projectmodel.6.2.1.nupkg'.
2022-07-29T04:04:17.4020967Z Response status code does not indicate success: 503 (Service Unavailable).

[MattGal]

@dotnet/dnceng do we have appropriate retries here?

2022-07-29T04:04:17.4016275Z Failed to download package 'NuGet.ProjectModel.6.2.1' from 'https://pkgs.dev.azure.com/dnceng/9ee6d478-d288-47f7-aacc-f6e6d082ae6d/_packaging/45bacae2-5efb-47c8-91e5-8ec20c22b4f8/nuget/v3/flat2/nuget.projectmodel/6.2.1/nuget.projectmodel.6.2.1.nupkg'.
2022-07-29T04:04:17.4020967Z Response status code does not indicate success: 503 (Service Unavailable).

.NET Core SDK 7.0.100-preview.5.22307.18 has the "product-ified" version of the retry logic I did last year so I am pretty sure lots of retry happened here. These logs are full of so many 503s and 400s that there must have been a pretty major blip for package feeds, but oddly https://status.dev.azure.com/_history is empty for the past week (it actually shows a single degradation, in pipelines, in Europe... in the entire last month so I am suspicious there).