Upgrade dependencies and follow nixos-25.05

The main reason for upgrading is that the previously provided GCC 13.3 doesn't support the latest RISC-V vector calling convention. (Where "support" is a bit euphemistic. GCC 13 and 14 pass vectors very differently to functions. GCC 13 passes all vector arguments on stack while GCC 14 is aligned with the current edition of the standard: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc#standard-vector-calling-convention-variant)

However, in general it doesn't hurt to:

• Follow a stable channel (nixos-25.05)
• Upgrade the dependencies in flake.lock