dotnetcore-sdk-bin updates (may 2020 updates, .net 5.0 support)

[ngg]

there are 4 changes:

• envvar setting for older versions
• 3.0 is no longer ~amd64 because it has reached EOL
• may 2020 updates: 2.1 is updated to 2.1.806, 3.1 is updated to 3.1.300
• 5.0.100-preview.4 is added

[ngg]

@cnd Can you please check this PR?

[ngg]

@cnd can I do something to get this merged? Is there something you do not like?

[cnd]

@ngg yes, it's very hard to merge PR with many commits because of very awkward git policy. (I don't like to add sign-off to every commit like a monkey during interactive rebase)

to make live easier I should move that repository from "official" but than all users will need to migrate

for now maybe squash all commits into one for future Pull Requests

[ngg]

Ok, in the future I'll create separate PRs for multiple commits or squash them together if that makes sense. Should I also add signed-off-by lines like when I make PRs to the main gentoo repo?

[cnd]

yes it does but for that I also should request for infra to register you as official collaborator with push access