ci: pin ubuntu to 20.04 to avoid glibc errors

ref: https://github.com/paradigmxyz/reth/actions/runs/5486255403/jobs/9996118421
ref: https://github.com/foundry-rs/foundry/issues/3827

Compiling serde v1.0.164
error: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.25' not found (required by /target/maxperf/deps/libserde_derive-6e9709c79bb5c7fa.so)
   --> /cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.164/src/lib.rs:340:1
    |
340 | extern crate serde_derive;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ name: ci
 jobs:
   lint:
     name: code lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout sources
         uses: actions/checkout@v3
@@ -40,7 +40,7 @@ jobs:
 
   doc-lint:
     name: doc lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - name: Install toolchain
@@ -51,7 +51,7 @@ jobs:
 
   grafana-lint:
     name: grafana lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - name: Check dashboard JSON with jq
@@ -62,7 +62,7 @@ jobs:
   lint-success:
     if: always()
     name: lint success
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: [lint, doc-lint, grafana-lint]
     steps:
       - name: Decide whether the needed jobs succeeded or failed