[ci] Restore ubuntu-focal build configuration

Author: kateinoigakukun

.github/scripts/build-matrix.rb

@@ -2,19 +2,18 @@
 require "optparse"
 
 BASE_MATRIX_ENTRIES = [
-# DEPRECATED: GitHub Actions won't support Ubuntu 18.0.4 faimily anymore
-#  {
-#    "build_os": "ubuntu-18.04",
-#    "agent_query": "ubuntu-18.04",
-#    "target": "ubuntu18.04_x86_64",
-#    "container": "ghcr.io/swiftwasm/swift-ci:main-ubuntu-18.04",
-#    "run_stdlib_test": true,
-#    "run_full_test": false,
-#    "run_e2e_test": true,
-#    "build_hello_wasm": true,
-#    "clean_build_dir": false,
-#    "free_disk_space": true
-#  },
+  {
+    "build_os": "ubuntu-18.04",
+    "agent_query": "ubuntu-20.04",
+    "target": "ubuntu18.04_x86_64",
+    "container": "ghcr.io/swiftwasm/swift-ci:main-ubuntu-18.04",
+    "run_stdlib_test": true,
+    "run_full_test": false,
+    "run_e2e_test": true,
+    "build_hello_wasm": true,
+    "clean_build_dir": false,
+    "free_disk_space": true
+  },
   {
     "build_os": "ubuntu-20.04",
     "agent_query": "ubuntu-20.04",