fix(R buildpack): Use R directly in `RenvLayer`
fix(R buildpack): Use R directly in `RenvLayer`

Avoids potential errors when using the patched `Rscript` indirection.

stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila