keep attributes when adding computed columns, ungroup() is too strong

[romainfrancois]

closes #5760

library(dplyr, warn.conflicts = FALSE)

m <- mtcars
attr(m, "foo") <- "a"
m <- dplyr::group_by(m, vs)
attr(m, "foo")
#> [1] "a"
m <- dplyr::group_by(m, cyl, .add = TRUE)
attr(m, "foo")
#> [1] "a"

^{Created on 2021-02-16 by the reprex package (v0.3.0)}

ungroup() wipes all attributes, but dplyr_reconstruct() would restore @groups which we don't want. Not sure this can be abstracted away ?