fix: create vendor tarball during propose_downstream

Add post-modifications action to create the vendor tarball and copy it to
the downstream repository before Packit uploads to the lookaside cache.
This ensures both Source0 and Source1 are available during propose_downstream.

Follows the same pattern used by go-fdo-server and go-fdo-client.

Signed-off-by: Paul Whalen <pwhalen@fedoraproject.org>

Author: Packit Build

.packit.yaml

@@ -36,6 +36,16 @@ actions:
   - "sed -i '/^License:/a\\%global debug_package %{nil}' greenboot-rs.spec"
   - bash -c "git config user.name 'Packit Build' || true"
   - bash -c "git config user.email 'packit@fedoraproject.org' || true"
+  post-modifications:
+  - |
+    bash -exc '
+    cargo vendor vendor
+    tar -cJf "greenboot-rs-${PACKIT_PROJECT_VERSION}-vendor-patched.tar.xz" vendor
+    rm -rf vendor
+    if [ -n "${PACKIT_DOWNSTREAM_REPO}" ]; then
+      cp "greenboot-rs-${PACKIT_PROJECT_VERSION}-vendor-patched.tar.xz" "${PACKIT_DOWNSTREAM_REPO}/"
+    fi
+    '
 
 jobs:
 - job: copr_build