graphics/openexr: update to bugfix release v3.2.4

ChangeLog: https://github.com/AcademySoftwareFoundation/openexr/blob/main/CHANGES.md#version-324-march-26-2024
ChangeLog: https://github.com/AcademySoftwareFoundation/openexr/blob/main/CHANGES.md#version-323-march-6-2024

Switch from auto-generated tarballs to those that use sigstore,
verify that -- see
https://github.com/AcademySoftwareFoundation/openexr/blob/main/SECURITY.md#signed-releases
and shuffle variables accordingly, including DISTFILES, EXTRACT_ONLY and NO_WRKSUBDIR updates.