chore: Run CI & downstreams on ARM linux (#923)

Author: jbelkins

.github/workflows/continuous-integration.yml

@@ -172,10 +172,13 @@ jobs:
 
   linux-ci:
     if: github.repository == 'smithy-lang/smithy-swift' || github.event_name == 'pull_request'
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.runner }}
     strategy:
       fail-fast: false
       matrix:
+        runner:
+          - ubuntu-24.04
+          - ubuntu-24.04-arm
         swift:
           - 5.9-focal
           - 6.0-jammy
@@ -202,10 +205,13 @@ jobs:
 
   linux-downstream:
     if: github.repository == 'smithy-lang/smithy-swift' || github.event_name == 'pull_request'
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.runner }}
     strategy:
       fail-fast: false
       matrix:
+        runner:
+          - ubuntu-24.04
+          - ubuntu-24.04-arm
         swift:
           - 5.9-focal
           - 6.0-jammy