Revert "ci: slightly simplify workflow definition"

This reverts commit 66d279f.

While the change worked, it lead to workflows showing a different and
uglier name on the GitHub web UI, which was not worth it when compared
to the DRY benefits.

.github/workflows/ci.yml

@@ -54,6 +54,15 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        target:
+          - x86_64-unknown-linux-gnu
+          - x86_64-unknown-linux-musl
+          - x86_64-pc-windows-gnu
+          - aarch64-unknown-linux-gnu
+          - aarch64-unknown-linux-musl
+          - x86_64-apple-darwin
+          - aarch64-apple-darwin
+
         include:
           - target: x86_64-unknown-linux-gnu
             host-target: x86_64-unknown-linux-gnu