build: factor out `pluginarchive` function to `src/Makefile.common.inc`

A subsequent patch will need to bundle a plugin in a test executable,
requiring the same command line used by the CLI to bundle plugins in the
`babeltrace2` executable, when `--enable-built-in-plugins` is used.
Move the `pluginarchive` "function" and `PLUGINS_PATH` definition to a
new file, `src/Makefile.common.inc`, and include that file from
`src/cli/Makefile.am`.

Change-Id: I36a8d6191deb82e5bceab3f5b038f69626ecdfe2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11830
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>