- Use {pak} for pkg installation and binaries on Linux

[pat-s]

• {pak} seems stable enough meanwhile to build on it
• Binary pkgs are used on Linux and macOS in favor of source installs. A functioning toolchain for source installs is still provided and stays major feature
• The switch to {pak} allowed to reduce many OS-specific parts in the YAML

This change will live in the default branch for some time for testing before templates are updated.

Changes

• Replaced {remotes} with {pak} in all installation calls
• Removed system lib dependency installation stages as {pak} is able to do so on its own - at least for the pkgs which have them listed in the DESCRIPTION file. Additional deps need to be installed in a custom block anyway
• Removed global env var SDKROOT as the workaround is no longer needed
• Removed global env var R_REMOTES_NO_ERRORS_FROM_WARNINGS as {remotes} is no longer used
• Removed if condition on "after_success" stages as {covr} is triggered by the matrix item codecov: true|false
• step_install_deps(), step_install_github(), step_install_cran(): remove arguments type and repo as {pak} does not use them

[pat-s]

Existing YAMLs seem to work fine with the new approach, merging and testing.