Fix duplicate attachment of sources artifact

The previous configuration caused the sources to be attached twice
via two different goals: once using jar-no-fork, and once using jar.

The reason this happened is because pom-scijava-base specified
<id>attach-sources</id>, which is aligned with the super POM, which
caused the two executions to be _concatenated_, not _replaced_ as I
believed would occur.

With this change, the following commands both look good to me now:

  mvn help:effective-pom
  mvn -Prelease-profile help:effective-pom

Both of them now have two executions:

* attach-sources from the super POM, overridden to <phase>none</phase>.
* attach-sources-jar from pom-scijava-base, using <goal>jar</goal>.

With luck, this will make releases of SciJava components work better.
The only thing I don't understand is why _any_ releases have been
working at all -- in theory, this problems breaks all release builds...

Author: ctrueden

pom.xml

@@ -470,14 +470,18 @@
 					<version>${maven-source-plugin.version}</version>
 					<!-- Build source artifact in addition to main artifact. -->
 					<executions>
+						<!--
+						NB: Attach sources with jar goal, not jar-no-fork, to avoid a bug
+						where the -sources JAR gets generated and deployed twice. See:
+						https://issues.apache.org/jira/browse/MNG-5939
+						-->
 						<execution>
 							<id>attach-sources</id>
+							<phase>none</phase>
+						</execution>
+						<execution>
+							<id>attach-sources-jar</id>
 							<goals>
-								<!--
-								NB: We use jar rather than jar-no-fork to avoid a bug where
-								the -sources JAR gets generated and deployed twice. See:
-								https://issues.apache.org/jira/browse/MNG-5939
-								-->
 								<goal>jar</goal>
 							</goals>
 						</execution>