ropensci/targets: tar_igraph()

targets 1.12.0 Avoid loading non-targets in tar_load(everything()) (#1529, @pitakakariki). For internal consistency, return NA_character_ from tar_path_target() when format is "file" (#1532, @noamross). Add tar_option_unset() (#1521, @noamross). Use a new TAR_ACTIVE environment variable instead of a field of the tar_runtime object for tar_runtime() (https://github.com/ropensci/tarchetypes/issues/232, @lgaborini). Add tar_igraph() and recommend it along with igraph::find_cycle() for debugging (#1562, @tylermorganwall). Avoid the unclean shutdown message in the clustermq multi-process scheduler by calling cleanup() repeatedly until it returns TRUE. Uses exponential backoff to avoid excessive CPU load. Use sequential controller in covr. Ensure hash stability of vector slices in R >= 4.6.0 (#1566). Use a dummy send instead of send_wait() to work around https://github.com/mschubert/clustermq/issues/340 (#1566). Set strict = TRUE by default in tidyselect::eval_select() to make tidyselect interfaces consistent with dplyr::select() (except the exclude argument of graph functions, which need to allow names to not exist in the list of choices) (#1563, @statzhero).