Use envvar to regenerate tests

Author: lionel-

tests/testthat/mock-pkg/tests/testthat/test-failed.R

@@ -5,7 +5,7 @@ library("vdiffr")
 p1_orig <- ggplot2::ggplot(mtcars, ggplot2::aes(disp)) + ggplot2::geom_histogram()
 p1_fail <- p1_orig + ggplot2::geom_vline(xintercept = 300)
 
-maintenance <- FALSE
+maintenance <- Sys.getenv("VDIFFR_REGENERATE_TESTS") == "yes"
 skip_if_maintenance <- function() {
   if (maintenance) {
     skip("maintenance")