Fix SciJava POM's scijava-common.groupId property

Thinko: if we use ${project.groupId} in the <properties> section of
the SciJava POM, it will be interpolated to the current project's
groupId in child POMs, not necessarily the scijava-common groupId...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>