cirrus: update alpine, make jammy use rustup to get Rust>=1.85

See https://github.com/fish-shell/fish-shell/pull/11961#discussion_r2439033959

I didn't find a good way to add ~/.cargo/bin to $PATH only for
fishuser, so hardcode it for all users for now.  The default rustup
behavior (source it in ~/.bashrc) only works for interactive shells,
so it doesn't work in CI.

I updated the images manually for now, because
I'm not sure I want to test on master (see
https://github.com/fish-shell/fish-shell/pull/11884 and
https://github.com/fish-shell/fish-shell/pull/11961#discussion_r2439033959)

	sed -i '/if: github.repository_owner/d' .github/workflows/build_docker_images.yml
	git commit
	# Upload new docker images.
	repo_owner=krobelus
	git push $repo_owner HEAD:tmp
	gh --repo=$repo_owner/fish-shell \
	    workflow run --ref=tmp \
	    .github/workflows/build_docker_images.yml
	# https://github.com/$repo_owner/fish-shell/actions/workflows/build_docker_images.yml
	sleep 3m
	# Enable Cirrus CI on fork.
	sed -i "s/OWNER == 'fish-shell'/OWNER == '$repo_owner'/g" .cirrus.yml
	git commit
	git push $repo_owner HEAD:tmp

Part of #11961

.github/workflows/build_docker_images.yml

@@ -30,6 +30,8 @@ jobs:
         include:
           - os: ubuntu-latest
             target: alpine
+          - os: ubuntu-latest
+            target: ubuntu-oldest-supported
           - os: ubuntu-latest
             target: centos9
           - os: ubuntu-latest