Run host-toolchain CI jobs on ubuntu-26.04

GitHub moves ubuntu-latest to Ubuntu 26.04 between 2026-10-19 and
2026-11-19 (actions/runner-images#14748). Run the jobs that use the
runner's own compilers and tools on ubuntu-26.04 now to find breakage
before the rollout. Container jobs are unaffected.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/check_amalgamation.yml

@@ -37,7 +37,7 @@ jobs:
           path: pr/
 
   check:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     env:
       MAIN_DIR: ${{ github.workspace }}/main
       INCLUDE_DIR: ${{ github.workspace }}/main/single_include/nlohmann