image: Prevent replacing underscores with dashes in image name tag

If git tag contains an underscore, we need to preserve it in output image file name for the test purposes e.g.: iowrt-7.4.0alpha1_dev-6-g25cfceed-x-y-z.pkgtb

Change still replaces spaces to underscores in VERSION_CODE even though it is unlikely to contain spaces as is created from a git describe output, but theoretically may contain spaces when version string is forced by user.

refs #13463