Set up a nightly build matrix for valgrind-based SCA

[reneme]

I feel we should build a nightly test matrix that runs the valgrind suite on a wider spectrum of compilers, targets and optimization levels, in the hope that this increases the signal of such vulnerabilities. Or at least see if we can build regression tests based on our existing tools.

Originally posted by @reneme in https://github.com/randombit/botan/pull/4395#issuecomment-2423824723