pmbootstrap log: use tail -F, not -f (MR 1982)

Let tail attempt to open the file again, if it becomes inaccessible.
This is useful, when writing a reproducer that deletes pmbootstrap's
log.txt while at the same time running 'pmbootstrap log'.

(027724) [17:57:34] Done
tail: '/home/user/.local/var/pmbootstrap/log.txt' has become inaccessible: No such file or directory
tail: '/home/user/.local/var/pmbootstrap/log.txt' has appeared;  following new file
(003493) [17:57:35] % cd /home/user/.local/var/pmbootstrap/cache_git/pmaports; git remote -v

pmb/helpers/frontend.py

@@ -451,7 +451,7 @@ def work_migrate(args):
 def log(args):
     if args.clear_log:
         pmb.helpers.run.user(args, ["truncate", "-s", "0", args.log])
-    pmb.helpers.run.user(args, ["tail", "-f", args.log, "-n", args.lines],
+    pmb.helpers.run.user(args, ["tail", "-F", args.log, "-n", args.lines],
                          output="tui")