CMake - the project is never up-to-date on `IMATH_INSTALL` enabled because of timestamp issues

[AnastaZIuk]

Issue:

• If IMATH_INSTALL option is enabled ZERO_CHECK target will always rerun CMake when the Imath target is requested to be built because the export file seems to be created at configure time as "dummy file" and later export is called causing the file to be overwritten at generate time resulting in incorrect timestamp checks

1>Build started 9/18/2024 11:47:37 AM.
1>Target PrepareForBuild:
1>  Structured output is enabled. The formatting of compiler diagnostics will reflect the error hierarchy. See https://aka.ms/cpp/structured-output for more details.
1>Target InitializeBuildStatus:
1>  Creating "x64\Debug\ZERO_CHECK\ZERO_CHECK.tlog\unsuccessfulbuild" because "AlwaysCreate" was specified.
1>  Touching "x64\Debug\ZERO_CHECK\ZERO_CHECK.tlog\unsuccessfulbuild".
1>Target PreBuildEvent:
1>  Checking File Globs
1>Target CustomBuild:
1>  1>Checking Build System
1>  CMake is re-running because D:/repositories/DevshGraphicsProgramming/Nabla/build/dynamic/3rdparty/openexr/CMakeFiles/generate.stamp is out-of-date.
1>    the file 'D:/repositories/DevshGraphicsProgramming/Nabla/build/dynamic/3rdparty/imath/config/ImathTargets.cmake'
1>    is newer than 'D:/repositories/DevshGraphicsProgramming/Nabla/build/dynamic/3rdparty/openexr/CMakeFiles/generate.stamp.depend'
1>    result='-1'

Proposed solution:

• remove the "dummy file" file write request, it's export job to write it

[AnastaZIuk]

of course turning IMATH_INSTALL off when building from sources is a workaround (the file doesn't change content for CMake iterations)