Fix maven-assembly-plugin skipAssembly leaking into every execution

Root pom.xml wired <skipAssembly>${pkg.skip.zip}</skipAssembly> at the
plugin-level <configuration> of maven-assembly-plugin inside the
always-active `packaging` profile's <pluginManagement>. Maven merges
plugin-level <configuration> into every execution of that plugin, so
-Dpkg.skip.zip=true (and the -Dpkg.skip=true alias that activates it)
suppressed any maven-assembly-plugin execution across the reactor -
not only the intended Windows ZIP execution.

In CE lts-4.2/4.3 this is latent (no CE module declares a non-ZIP
assembly execution), but it breaks downstream forks that do. PE's
rule-node-twilio-sms, for instance, declares a custom make-assembly
execution producing the classified -rule-node.jar consumed by
application's copy-pe-rule-nodes step; under -Dpkg.skip.zip=true that
assembly silently became a no-op and the downstream build failed to
resolve the classified artifact.

tools/pom.xml already sidesteps this via `combine.self="override"` on
its own <pluginManagement> - earlier evidence that the placement was
fragile.

Move <skipAssembly> into the `assembly` execution's own <configuration>
so it scopes only to the Windows ZIP execution.

Verified via mvn help:effective-pom on application/: with the fix,
<skipAssembly>true</skipAssembly> no longer appears at plugin-level
<configuration>, only inside the `assembly` <execution>.

pom.xml

@@ -618,7 +618,6 @@
                             <groupId>org.apache.maven.plugins</groupId>
                             <artifactId>maven-assembly-plugin</artifactId>
                             <configuration>
-                                <skipAssembly>${pkg.skip.zip}</skipAssembly>
                                 <finalName>${pkg.name}</finalName>
                                 <descriptors>
                                     <descriptor>${main.dir}/packaging/${pkg.type}/assembly/windows.xml</descriptor>
@@ -631,6 +630,9 @@
                                     <goals>
                                         <goal>single</goal>
                                     </goals>
+                                    <configuration>
+                                        <skipAssembly>${pkg.skip.zip}</skipAssembly>
+                                    </configuration>
                                 </execution>
                             </executions>
                         </plugin>