[GR-48515] Enforce .exports files gets safely created for a cmd-launcher.

[graalvmbot]

This PR changes the creation time of the .exports-files to mx build time.
Now after running mx build we will already have:

<graalvm-home>/lib/svm/native-image-configure.cmd
<graalvm-home>/lib/svm/native-image-configure.export-list
<graalvm-home>/lib/svm/native-image.cmd
<graalvm-home>/lib/svm/native-image.export-list
...

This prevents the concurrent writing bug of the same.exportsfile on concurrent first use of native-image.cmd that could happen with the previous solution.