[Build] Don't resign already signed jars

Reduce the number of actual signing operations by not re-signing
artifacts (in the I-build) are already signed, usually because they have
been 'baseline-replaced'.

The baseline replacement is performed first the 'p2-metadata-default'
goal executed by default and before the jar-signing.
The subsequent explicit invocation of the p2-metadata (non-default) goal
is again necessary to update the p2 metadata (like artifact size) after
the jar has been altered by the signing operation.

Fixes #2134

eclipse-platform-parent/pom.xml

@@ -837,6 +837,7 @@
               </execution>
             </executions>
             <configuration>
+              <resigningStrategy>DO_NOT_RESIGN</resigningStrategy>
               <excludeInnerJars>${defaultSigning-excludeInnerJars}</excludeInnerJars>
               <!--
                The default timeout is 0 seconds which means "wait forever": https://www.eclipse.org/cbi/sitedocs/eclipse-jarsigner-plugin/sign-mojo.html#timeoutMillis