GMockConfig: switch the workaround to Marius's variant

Turns out, the fake add_custom_command() doesn't always work on projects with multiple tests - for some reason. And also turns out that Marius has been worked around this issue globally in Debian and Ubuntu for some time now.

So I essentially take his workaround, repackage it a bit nicely, and limit the workaround to just the affected versions only. And while we're at it, set the upper bound on the affected versions since the fix now landed in CMake 3.28.4.