how to move "wkhtmltopdf" in the right place with terminal commands
There are 2 answers ☺:

Don't do it!

The wkhtmltopdf wasn't installed in /usr/bin. It shouldn't.
This directory is reserved to standard user Unix commands.

Check carefully where brew installed the last rc version.

The following command will tell you the truth:

type wkhtmltopdf


Most probably this installation directory is:

/local/bin
/usr/local/bin
/opt/bin
...


How to do it

Once you have located where brew installed your not working rc,
let's say it's /local_installdir.
Once you have built the previous version in a directory,
let's say it's /local_builddir
here is how to use mv to place this previous version in place:

cd /local_builddir
mv wkhtmltopdf /local_installdir


If you get a permission denied error message on this operation,
then you will have to prefix this same command with the usual
sudo:

/usr/bin/sudo mv wkhtmltopdf /local_installdir