Fix path to look for Mylyn Wikitext in case of shared installations

Fix path to look for Mylyn Wikitext in case of shared installations

${eclipse.home} works correctly in the case where the Mylyn plugins are under
the Eclipse installation but it does not work in the case of bundle pools,
for example /home/user/.p2/pool/plugins. Instead, ${osgi.syspath} can be used.

Change-Id: Ife437c6efe1b7677210cd5cb444cf689c4384874
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/39626
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-by: Hudson CI