Use a consistent ubuntu across all Linux CI jobs.

Rollout of ubuntu-24.04 as ubuntu-latest broke our Python CI due
to newer and stricter GCC. Mitigate by using ubuntu-20.04, matching
what we currently use for the PyPI release.

.github/workflows/ci_python.yml

@@ -18,7 +18,7 @@ jobs:
         #  1. they removed support for python 3.8 & 3.9 on macos-14 (now macos-latest)
         #    - See: https://github.com/actions/setup-python/issues/696#issuecomment-2071769156
         #  2. `test_pose` fails on macos-13 and macos-14 (but not macos-12)
-        os: [ubuntu-latest, macos-12]
+        os: [ubuntu-20.04, macos-12]
         python-version: ['3.8', '3.9', '3.10', '3.11', '3.12']
     steps:
     - uses: actions/checkout@v4

.github/workflows/docs.yml

@@ -12,7 +12,7 @@ on:
 jobs:
   deploy:
     name: Deploy to GitHub Pages
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
         with:

.github/workflows/format.yml

@@ -14,7 +14,7 @@ concurrency:
 
 jobs:
   pre-commit-check:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v3

.github/workflows/pypi-release.yml

@@ -41,7 +41,7 @@ jobs:
 
   build_sdist:
     name: Build source distribution
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v4
       - name: Build sdist
@@ -52,7 +52,7 @@ jobs:
 
   upload_pypi:
     needs: [build_wheels, build_sdist]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     if: github.event_name == 'release' && github.event.action == 'published'
     steps:
       - uses: actions/download-artifact@v3

.github/workflows/release.yml

@@ -13,7 +13,7 @@ jobs:
   # Based on https://dev.to/eugenebabichenko/automated-multi-platform-releases-with-github-actions-1abg
   create_release:
     name: Create Release and Provide Upload URL.
-    runs-on: ubuntu-latest # OS to create release, not for build
+    runs-on: ubuntu-20.04 # OS to create release, not for build
     outputs:
       # This job will provide URL for build jobs to use for uploading assets
       upload_url: ${{ steps.create_release.outputs.upload_url }}