tweak pkgdown workflow to install usethis

[maelle]

cf https://github.com/r-lib/pkgdepends/pull/224/files

If usethis were installed when building the pkgdown website (of pkgdown 😅 ) there'd be a link on use_pkgdown() on https://pkgdown.r-lib.org/dev/articles/pkgdown.html (and there are other occurrences of usethis functions)

Now I don't understand why the released website has a link to usethis' function.

[maelle]

The previous workflow used to install devtools hence the usethis auto-linking at the time 48dabb1#diff-eb3d51fe47cf000aab65c6172c06d12a34ae765221529a05451295d5e9149e80

[maelle]

r-lib/actions#264