Enable release mode builds (#552)

### Motivation:

Some errors do not show up in debug builds. Enabling release mode builds
improves the CI coverage.

### Modifications:

Enable release mode builds for pull requests and scheduled builds on
main.

### Result:

Improved CI coverage.

Author: josephnoir

.github/workflows/main.yml

@@ -59,3 +59,7 @@ jobs:
     static-sdk:
         name: Static SDK
         uses: apple/swift-nio/.github/workflows/static_sdk.yml@main
+
+    release-builds:
+        name: Release builds
+        uses: apple/swift-nio/.github/workflows/release_builds.yml@main

.github/workflows/pull_request.yml

@@ -66,3 +66,7 @@ jobs:
     static-sdk:
         name: Static SDK
         uses: apple/swift-nio/.github/workflows/static_sdk.yml@main
+
+    release-builds:
+        name: Release builds
+        uses: apple/swift-nio/.github/workflows/release_builds.yml@main