installer: utils: Dump command output to syslog when testing.

installer: utils: Dump command output to syslog when testing.

When debugging the installation tests, it can be very handy to be able to read
"run-command" output, for instance when executing "guix system init".

Introduce a new "invoke-with-log" procedure that is able to log a command
standard and error outputs to the syslog. Use it, only when running the
installation tests, to dump "run-command" output.

* gnu/installer/utils.scm (open-pipe-with-stderr, invoke-with-log): New
procedures,
(invoke-log-port): new variable,
(run-command): move to the end of the file and use invoke-with-log when
running the installation tests.