Fix ppc64 build

[Johannestegner]

There is currently a (constant) error on building latest nginx for ppc64 which makes the CI scripts fail.
This seems to apply to both image versions and is only effecting ppc64.

Alpine have been updated to 3.21 on both build and image, so it's not due to outdated compiler or similar.

Possible temporary solution (to allow building image) is to disable ppc64, this just to make sure that latest pushes are clear of the vulnerabilities that the current latest image have.

Ccache should be clared to make sure that its not an error which is due to broken cache.
Further investigation is needed to see what makes the build step break though.

Final lines of build log where failure happens:

checking for sig_atomic_t size ... 4 bytes
checking for socklen_t ... found
checking for in_addr_t ... found
checking for in_port_t ... found
checking for rlim_t ... rlim_t not found, int used
checking for uintptr_t ... uintptr_t not found, uint64_t used
checking for system byte ordering ... little endian
checking for size_t size ...
./configure: error: can not detect size_t size

[Johannestegner]

Currently the two failing architectures (ppc64 and x390s) are disabled in the build.