devel/jujutsu: fix build with ports tree checked out by jujutsu

Fix issue reported in PR 295250:

My poudriere ports tree is a null mounted jj repo (git
colocated). Building docs fails, because it calls

jj util install-man-pages

which evidently tries to access

/usr/ports/.jj/repo/config-id

because it's being run from inside a jj repo.

There must not be a .jj directory in the directory were jj is
run or in any parent directory to prevent an error abort:

Internal error: Failed to determine the secure config for a repo

The fix suggested by the reporter is to execute the just built jj
command in a temporary directory to build the docs.

PR: 295250
Reported by: pat@patmaddox.com