Add R CMD Check workflow back for duckdb src PRs

[Tmonster]

The duckdb R package is no longer in the duckdb source. We still want to check with every PR that the duckdb R package can built with no errors. This PR will fix that by running the r-cmd-check on duckdb PRs. Tests are not run, only building the R package is checked

the workflow is as follows

1. checkout duckdb pull request branch
2. checkout duckdb-r source
3. Setup R
4. Install duckdb package dependencies
5. update the duckdb-r source with the source from the pull request branch (using the ./vendor.sh script)
6. Run R CMD build . and R CMD check .
7. sanity check that the library can be imported.

[Tmonster]

Any notes about the platforms that this runs on? The check also takes a while >20 min, so I'm not sure if we want to run this on 3 platforms for every PR.

[krlmlr]

Yeah, I'm also working on shortening the build times in duckdb-r.

[krlmlr]

We can enable parallel builds:

      - name: Build in parallel
        run: |
          mkdir -p ~/.R
          echo 'MAKEFLAGS = -j2' >> ~/.R/Makevars
      shell: bash

[hannes]

I think one platform (Linux?) is fine here

[hannes]

@Mytherin any objections?

[Mytherin]

LGTM