fix: use curl+gpg for deadsnakes PPA, arm64 base back to ubuntu:24.04

Ubuntu 26.04 ships Python 3.14, not 3.13. Reverted arm64 to ubuntu:24.04.
Both architectures now add the deadsnakes PPA by fetching the GPG key via
curl and piping through gpg --dearmor — no gpg-agent, safe under QEMU.
Removes the per-arch conditional and ARG TARGETARCH dependency in RUN commands.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

uv.lock

@@ -2193,7 +2193,7 @@ wheels = [
 
 [[package]]
 name = "winnow"
-version = "0.2.6"
+version = "0.2.7"
 source = { editable = "." }
 dependencies = [
     { name = "croniter" },