Add static SDK CI workflow (#55)

Add static SDK CI workflow which runs on commits to PRs, merges to main
and daily on main.

---------

Co-authored-by: George Barnett <gbarnett@apple.com>

.github/workflows/main.yml

@@ -42,3 +42,7 @@ jobs:
     with:
       name: "Plugin tests"
       matrix_string: '${{ needs.construct-plugin-tests-matrix.outputs.plugin-tests-matrix }}'
+
+  static-sdk:
+    name: Static SDK
+    uses: apple/swift-nio/.github/workflows/static_sdk.yml@main

.github/workflows/pull_request.yml

@@ -58,3 +58,7 @@ jobs:
     with:
       linux_5_9_enabled: false
       linux_5_10_enabled: false
+
+  static-sdk:
+    name: Static SDK
+    uses: apple/swift-nio/.github/workflows/static_sdk.yml@main