Update github actions to use r-lib actions v2

[dschlaep]

• see https://github.com/r-lib/actions/releases/tag/v2

• also install makeindex to avoid error on "'"makeindex"' not found"

[dschlaep]

This currently fails while attempting to installing rSOILWAT2.

The r-lib v2 actions now use pak to install R packages on which rSW2funs relies on -- previously, they were installed via remotes.

My guess is that pak v0.2.1 fails to install rSOILWAT2 because it (respectively, pkgdepends v0.2.0) currently does not resolve git submodules -- something that remotes does.

[codecov]

Codecov Report

Merging #4 (ed1fc03) into main (764eced) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main       #4   +/-   ##
=======================================
  Coverage   79.84%   79.84%           
=======================================
  Files           7        7           
  Lines        1836     1836           
=======================================
  Hits         1466     1466           
  Misses        370      370           

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