fix(ci): correct release matrix target triple for ubuntu (#589)

The ubuntu-latest matrix entry had its target set to
'blacksmith-2vcpu-ubuntu-2404', which is a runner label — not a valid
Rust target triple. This causes 'cargo build --target' to fail or
produce artifacts for the wrong architecture.

Replace with 'x86_64-unknown-linux-gnu', the correct Rust target triple
for the ubuntu runner.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/release.yml

@@ -21,7 +21,7 @@ jobs:
             matrix:
                 include:
                     - os: ubuntu-latest
-                      target: blacksmith-2vcpu-ubuntu-2404
+                      target: x86_64-unknown-linux-gnu
                       artifact: zeroclaw
                     - os: macos-latest
                       target: x86_64-apple-darwin