CI: run the i686 container on Ubuntu 24.04

Since ubuntu-latest moved to 26.04, CI Linux (i686) stops at checkout:
git fetch cannot resolve github.com from the 32-bit xenial image, ten
tries in a row, and the public resolvers the job passes with --dns do
not change that. The 64-bit container jobs on the same runners check
out and build. The job goes back to ubuntu-24.04, where it last
passed, like the mipsel lanes.

.github/workflows/Linux.yml

@@ -31,7 +31,10 @@ concurrency:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    # The 32-bit xenial image resolves no host on an Ubuntu 26.04 runner,
+    # public resolvers or not: since ubuntu-latest moved, its runs stop at
+    # checkout. The other container jobs, all 64-bit, are unaffected.
+    runs-on: ubuntu-24.04
     timeout-minutes: 30
     container:
       image: reallibretroretroarch/libretro-build-i386-ubuntu:xenial-gcc9