ci: Upgrade ubuntu:25.04 for the PowerPC64LE test

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.

Author: tgross35

ci/docker/powerpc64le-unknown-linux-gnu/Dockerfile

@@ -1,5 +1,4 @@
-# FIXME(ppc): We want 25.04 but get SIGILLs
-ARG IMAGE=ubuntu:24.04
+ARG IMAGE=ubuntu:25.04
 FROM $IMAGE
 
 RUN apt-get update && \
@@ -13,6 +12,5 @@ ENV CARGO_TARGET_POWERPC64LE_UNKNOWN_LINUX_GNU_LINKER="$TOOLCHAIN_PREFIX"gcc \
     CARGO_TARGET_POWERPC64LE_UNKNOWN_LINUX_GNU_RUNNER=qemu-ppc64le-static \
     AR_powerpc64le_unknown_linux_gnu="$TOOLCHAIN_PREFIX"ar \
     CC_powerpc64le_unknown_linux_gnu="$TOOLCHAIN_PREFIX"gcc \
-    QEMU_CPU=POWER8 \
     QEMU_LD_PREFIX=/usr/powerpc64le-linux-gnu \
     RUST_TEST_THREADS=1