Pin CI jobs to Ubuntu 20.04 (#7359)

The release job was pinned to Ubuntu 20.04 to avoid glibc breakage in
#7290. This PR updates the CI
jobs to keep things consistent (it would sure be unpleasant if things
worked in CI but not at release time).

.github/workflows/ci.yml

@@ -11,7 +11,10 @@ jobs:
  strategy:
  fail-fast: true
  matrix:
- platform: [windows-latest, macos-latest, ubuntu-latest]
+ # Pinning to Ubuntu 20.04 because building on newer Ubuntu versions causes linux-gnu
+ # builds to link against a too-new-for-many-Linux-installs glibc version. Consider
+ # revisiting this when 20.04 is closer to EOL (April 2025)
+ platform: [windows-latest, macos-latest, ubuntu-20.04]
  rust:
  - stable
 
@@ -44,7 +47,7 @@ jobs:
  strategy:
  fail-fast: true
  matrix:
- platform: [windows-latest, macos-latest, ubuntu-latest]
+ platform: [windows-latest, macos-latest, ubuntu-20.04]
  style: [default, dataframe]
  rust:
  - stable
@@ -81,7 +84,7 @@ jobs:
  strategy:
  fail-fast: true
  matrix:
- platform: [ubuntu-latest, macos-latest, windows-latest]
+ platform: [ubuntu-20.04, macos-latest, windows-latest]
  rust:
  - stable
  py:
@@ -125,7 +128,7 @@ jobs:
  strategy:
  fail-fast: true
  matrix:
- platform: [windows-latest, macos-latest, ubuntu-latest]
+ platform: [windows-latest, macos-latest, ubuntu-20.04]
  rust:
  - stable