message: ci: Explicitly set Linux runner version, opt in to new stable.
  See also
  where it is explained that `ubuntu-latest` will soon point to
  something different.  That shouldn't affect us, but let's upgrade and
  pin a particular version while we're at it.  This way we shouldn't be
  surprised by changes in the future.
diff: @@ -11,7 +11,7 @@ on:
  jobs:
  spec:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  name: Check Makam Specification
  steps:
  - uses: actions/checkout@v2
  @@ -23,7 +23,7 @@ jobs:
  working-directory: ./experiments/makam-spec
  check:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  strategy:
  matrix:
  rust: ["1.42.0", "stable", "beta", "nightly"]
  @@ -42,7 +42,7 @@ jobs:
  command: check
  test:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  strategy:
  matrix:
  rust: ["1.42.0", "stable", "beta", "nightly"]
  @@ -61,7 +61,7 @@ jobs:
  command: test
  fmt:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  strategy:
  matrix:
  rust: ["1.42.0", "stable", "beta", "nightly"]
  @@ -82,7 +82,7 @@ jobs:
  args: --all -- --check
  # clippy:
  -# runs-on: ubuntu-latest
  +# runs-on: ubuntu-20.04
  # strategy:
  # matrix:
  # rust: ["1.42.0", "stable", "beta", "nightly"]
  @@ -103,7 +103,7 @@ jobs:
  # args: -- -D warnings
  book:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  name: Build and Test Book
  env:
  MDBOOK_VERSION: '0.4.4'
  