ci: run gem with sudo to install asciidoctor

ci: run gem with sudo to install asciidoctor

In a later patch, we will run Documentation job in GitHub Actions.
The job will run without elevated permission.

Run `gem` with `sudo` to elevate permission in order to be able to
install to system location.
This will also keep this installation in-line with other installation in
our Linux system for CI.

Signed-off-by: Johannes Schindelin <redacted>
[Danh: reword commit message]
Signed-off-by: Đoàn Trần Công Danh <redacted>
Signed-off-by: Junio C Hamano <redacted>