Configure R plugin for RProvider v2

[AndrewIOM]

RProvider v2 is compiled for .NET 5 and greater - a fix has just been applied to re-enable plugins.
Addresses #526

I have reconfigured the Deedle RProvider plugin to work with this latest RProvider version, compiled against net-5.0. This has been tested on a local RProvider source - the nuget package for this will shortly be updated. At that stage, I will reference it from here and remove the draft mark from this PR.

[zyzhu]

Awesome work. I've tested locally and it works like a charm. I'll merge this and clean up the build script and release a new version referencing RProvider 2.0.1. Or shall I wait till you release a newer version 2.0.2? I saw there are some breaking changes coming in the latest pull requests in RProvider.

[AndrewIOM]

@zyzhu Great! I've just pushed a version 2.0.2 incorporating the breaking change so you can change the RProvider version to that in Deedle. I noticed I missed a change in the push-master.yml. Changing this

      - name: Set R_HOME (for RProvider)
        run: echo "R_HOME=$(R RHOME)" >> "$GITHUB_ENV"
        shell: bash

to this

      - name: Set R environment variables (for RProvider)
        run:  |
          mkdir -p ~/.local/lib/R
          echo "R_LIBS_USER=~/.local/lib/R" >> "$GITHUB_ENV"
          echo "R_HOME=$(R RHOME)" >> "$GITHUB_ENV"

should fix the build error