fix some errors in R-package after upgrade to 1.0.7

[StrikerRUS]

• After the recent refactoring of Python finding routine reticulate fails to find Python from conda environment.

  echo "Sys.setenv(RETICULATE_PYTHON = '$CONDA_PREFIX/bin/python')" >> .Rprofile

  similarly to the same command on Windows fixes this. https://github.com/RGF-team/rgf/blob/f7407902de64f224109929d80ffb45c951b88278/.ci/r_tests_windows.ps1#L49

• roxygen2 starting from 7.1.0 version starts to produce references to new() and clone() methods. They are broken for some reason:

  LaTeX errors found:
  ! Undefined control sequence.
  l.152 \hypertarget
                  {method-new}{}
  The control sequence at the end of the top line
  of your error message was never \def'ed. If you have
  ! Undefined control sequence.
  l.231 \hypertarget
                  {method-clone}{}

  Here is the corresponding PR: https://github.com/r-lib/roxygen2/pull/1007. As a workaround I recreated Rd files with the roxygen2 7.0.2 which doesn't generate that links. Later I believe we will be able to make some refactoring of docs and make RGF package compatible with roxygen2 7.1.x. Also I saw a lot of warnings during roxygen2::roxygenise() which is not good and indicates that we should refactor docs ideally.