ci: make static built binary available

This uploads the results of make release step (static binary and
a source tarball) so that they are available. I am not very sure if
it's of any use, but at least one can download and play with a static
binary from any PR.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/validate.yml

@@ -156,3 +156,8 @@ jobs:
         sudo apt -qq install libseccomp-dev
     - name: make release
       run: make release
+    - name: upload artifacts
+      uses: actions/upload-artifact@v2
+      with:
+        name: release-${{ github.run_id }}
+        path: release/*