Add verbose log in GitHub Actions CI for make -f Makefile.doc (#4681)
Add verbose log in GitHub Actions CI for make -f Makefile.doc (#4681)

Our GitHub Actions CI is having some issues with `make -f Makefile.doc`
as the related changes that should have triggered the build didn't
work for the past 3 months (since march).

This PR adds verbose to GitHub Actions CI to see if we can get more information.

Note this PR just add `set -x` to show detailed step. Will not impact anything else.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
chrisohaver/coredns,coredns/coredns,yongtang/coredns