make sure downloaded artifacts are all in the same dir

It seems that when we request multiple artifacts at the same time,
they are by default placed in their own subdirectories. There's a
key to override this; this commit sets it.

.github/workflows/validate.yml

@@ -478,6 +478,7 @@ jobs:
         with:
           pattern: cabal-*
           path: binaries
+          merge-multiple: true
 
       - name: Create GitHub prerelease
         uses: softprops/action-gh-release@v2
@@ -502,6 +503,7 @@ jobs:
       with:
         pattern: cabal-*
         path: binaries
+        merge-multiple: true
 
     - run: |
         # bash-ism, but we forced bash above