Fix signing Windows executables

The v0.2-RC1 build failed on master:

20:57:34  [INFO] --- eclipse-winsigner-plugin:1.3.1:sign (sign) @ org.eclipse.escet.product ---
[...]
20:57:34  [ERROR] Failed to execute goal org.eclipse.cbi.maven.plugins:eclipse-winsigner-plugin:1.3.1:sign (sign) on project org.eclipse.escet.product: [Fri Jul 02 18:57:33 UTC 2021] Path '/home/jenkins/agent/workspace/ESCET_build_master/products/org.eclipse.escet.product/target/products/org.eclipse.escet.product/win32/win32/x86_64/eclipse.exe' does not exist or is not a file. It won't be signed. -> [Help 1]