[blazor-wasm] Ensure PDBArtifacts artifact is created

[dougbu]

• dotnet/aspnetcore-internal#3535
• avoid build warnings when publishing symbols in post-build

[pranavkm]

If you think this is the right fix. I'm not entirely sure why pdbs are missing though.

[dougbu]

Internal build: https://dev.azure.com/dnceng/internal/_build/results?buildId=558785&view=results

[dougbu]

https://dev.azure.com/dnceng/internal/_build/results?buildId=558785&view=artifacts&type=publishedArtifacts shows build created the PDBArtifacts artifact.

I forgot to create a default channel for my branch to ensure the missing artifact was the only problem causing the "issues" in our official builds. Tried again and also asked @JohnTortugo to execute darc add-build-to-channel --id 45804 --channel 'General Testing' for me, providing two chances to see symbol publication "succeed" (do nothing). Thanks @JohnTortugo❕

The real build (https://dev.azure.com/dnceng/internal/_build/results?buildId=558921) ran fine but still didn't try to push anything into the 'General Testing' feeds. Not sure why but maybe that channel doesn't exist in the older version of Arcade used in this branch (it's 143 commits behind 'blazor-wasm' and I built the branch directly instead of creating an internal PR)?

The promotion build https://dnceng.visualstudio.com/internal/_build/results?buildId=558940 succeeded without issues and definitely found the PDBArtifacts artifact (see https://dnceng.visualstudio.com/internal/_build/results?buildId=558940&view=logs&j=65907e9e-dc2e-508d-4ed4-c5ec62867dbe&t=27af53db-dc59-5d1e-3865-33df1e36904b&l=177). So, we're good😺

[dougbu]

Will merge this once I prepare no-op merge PRs for 'release/3.1' and 'master' branches.

[dougbu]

🆙📅 to move my git merge-base back to include a 'release/3.1' common ancestor. No functional changes included.

This update avoids marking ~65 'blazor-wasm' commits as being present in the 'release/3.1' branch.