unix/linux: switch to ubuntu 25.04, Go 1.25.1

Ubuntu 24.10 is not supported since July 2025 (and some of its apt repos
are apparently removed), and so mkall.sh no longer works (as of today).

Fix this by switching to (currently supported) Ubuntu 25.04.

NOTE that we can't switch to
 - Ubuntu 24.04 LTS release (see CL 618075);
 - Ubuntu 25.10 as it lacks mips support;
 - Debian Stable (13) as it also lacks mips support.

Also, bump Go to 1.25.1.

No changes in generated content after running

	GOOS=linux GOARCH=amd64 ./mkall.sh

Change-Id: I58a4856cda93a85af51f4a519c6e4e25f31a39b4
Reviewed-on: https://go-review.googlesource.com/c/sys/+/706915
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>