Avoid trying to sign redundant launchers that don't exist

[merks]

No description provided.

[merks]

The master build signed these:

[INFO] --- eclipse-winsigner:1.4.3:sign (sign) @ birt-report-all-in-one ---
[INFO] [Fri Jun 07 11:14:07 UTC 2024] Signing Windows executable '/home/jenkins/agent/workspace/build_master/build/birt-packages/birt-report-all-in-one/target/products/org.eclipse.birt.report.designer.all/win32/win32/x86_64/eclipse/birt.exe'
[INFO] [Fri Jun 07 11:14:09 UTC 2024] Signing Windows executable '/home/jenkins/agent/workspace/build_master/build/birt-packages/birt-report-all-in-one/target/products/org.eclipse.birt.report.designer.all/win32/win32/x86_64/eclipse/birtc.exe'b

But then failed because these no longer exist:

[ERROR] Failed to execute goal org.eclipse.cbi.maven.plugins:eclipse-winsigner-plugin:1.4.3:sign (sign) on project birt-report-all-in-one: [Fri Jun 07 11:14:13 UTC 2024] Path '/home/jenkins/agent/workspace/build_master/build/birt-packages/birt-report-all-in-one/target/products/org.eclipse.birt.report.designer.all/win32/win32/x86_64/eclipse/eclipsec.

But a PR build does no signing so will not test this PR.