message: Explicitly set Linux runner version, opt in to new stable
  See also actions/virtual-environments#1816, where it is explained that `ubuntu-latest` will soon point to something different.
diff: @@ -13,7 +13,7 @@ on:
  jobs:
  check:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  strategy:
  matrix:
  rust: ["1.44.0", "stable", "beta", "nightly"]
  @@ -32,7 +32,7 @@ jobs:
  command: check
  test:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  strategy:
  matrix:
  rust: ["1.44.0", "stable", "beta", "nightly"]
  @@ -51,7 +51,7 @@ jobs:
  command: test
  fmt:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  strategy:
  matrix:
  rust: ["1.44.0", "stable", "beta", "nightly"]
  @@ -72,7 +72,7 @@ jobs:
  args: --all -- --check
  clippy:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  strategy:
  matrix:
  rust: ["stable"]
  @@ -94,7 +94,7 @@ jobs:
  args: -- -D warnings
  book:
  - runs-on: ubuntu-latest
  + runs-on: ubuntu-20.04
  name: Build and Test Book
  env:
  MDBOOK_VERSION: '0.4.2'
  