agent: upgrade to ubuntu-latest for presubmit. (#2127)

diff --git a/.github/workflows/presubmit-agent.yml b/.github/workflows/presubmit-agent.yml
index 46e4a30..5be8b82 100644
--- a/.github/workflows/presubmit-agent.yml
+++ b/.github/workflows/presubmit-agent.yml
@@ -16,7 +16,7 @@
         build: [linux, macos, win-msvc]
         include:
         - build: linux
-          os: ubuntu-18.04
+          os: ubuntu-latest
           rust: nightly
           target: x86_64-unknown-linux-musl
         - build: macos