ci: Upgrade ubuntu:25.04 for the PowerPC64LE test

[tgross35]

Update the last remaining image.

For this to work, the QEMU_CPU=POWER8 configuration needed to be
dropped to avoid a new SIGILL. Doing some debugging locally, the crash
comes from an extswsli (per powerpc:common64 in gdb-multiarch) in
the ld64.so available with PowerPC, which qemu rejects when set to
power8. Testing a build with +crt-static hits the same issue at a
maddld in __libc_start_main_impl.

Rust isn't needed to reproduce this:

$ cat a.c
#include <stdio.h>

int main() {
        printf("Hello, world!\n");
}
$ powerpc64le-linux-gnu-gcc a.c
$ QEMU_CPU=power8 QEMU_LD_PREFIX=/usr/powerpc64le-linux-gnu/ ./a.out
qemu: uncaught target signal 4 (Illegal instruction) - core dumped
Illegal instruction

So the cross toolchain provided by Debian must have a power9 baseline
rather than rustc's power8. Alternatively, qemu may be incorrectly
rejecting these instructions (I can't find a source on whether or not
they should be available for power8). Testing instead with the -musl
toolchain and ppc linker from musl.cc works correctly.

In any case, things work with the default qemu config so it seems fine
to drop. The env was originally added in 5d164a4 ("fix the
powerpc64le target") but whatever the problem was there appears to no
longer be relevant.