ci: Move to Ubuntu 20.04 Actions runners

- The 18.04 runners are deprecated and going away in less than a month.
  Today was another scheduled brownout from GitHub
  (https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/),
  and they've been unmarked as required checks in the branch protections
  because the images are unavailable and the jobs were failing.
- However, we still need to test that Homebrew works correctly on
  non-latest Ubuntu versions so that we can test that we're correctly
  using brewed gcc/glibc, hence upgrading to 20.04 which is still old.

.github/workflows/tests.yml

@@ -264,9 +264,9 @@ jobs:
           - name: tests (Ubuntu 22.04)
             test-flags: --coverage
             runs-on: ubuntu-22.04
-          - name: tests (Ubuntu 18.04)
+          - name: tests (Ubuntu 20.04)
             test-flags: --coverage
-            runs-on: ubuntu-18.04
+            runs-on: ubuntu-20.04
           - name: tests (macOS 12)
             test-flags: --coverage
             runs-on: macos-12
@@ -354,8 +354,8 @@ jobs:
         include:
           - name: test default formula (Ubuntu 22.04)
             runs-on: ubuntu-22.04
-          - name: test default formula (Ubuntu 18.04)
-            runs-on: ubuntu-18.04
+          - name: test default formula (Ubuntu 20.04)
+            runs-on: ubuntu-20.04
           - name: test default formula (macOS 12)
             runs-on: macos-12
     steps: