CI: pin ubuntu runner to 20.04 to avoid sanitizer failures

[hellkite500]

Recently tests are failing with AddressSanitizer:DEADLYSIGNAL. A ctest timeout occurs and causes the test suite to fail. This does NOT occur when actually running the tests, but instead during the build/linking step. In the test CMakeLists, there is a call to

gtest_discover_tests

which defers test discovery until runtime instead of scanning source files. In effect, the causes the test executable to run with the flag --gtest_list_tests.
The cmake error points to this call as failing:

CMake Error at /usr/local/share/cmake-3.28/Modules/GoogleTestAddTests.cmake:112 (message):
2024-03-15T22:38:17.3140582Z   Error running test executable.
2024-03-15T22:38:17.3140590Z 
2024-03-15T22:38:17.3140878Z     Path: '/home/runner/work/ngen/ngen/cmake_build/test/test_bmi_c'
2024-03-15T22:38:17.3141033Z     Result: Process terminated due to timeout
2024-03-15T22:38:17.3141126Z     Output:
2024-03-15T22:38:17.3141215Z       
2024-03-15T22:38:17.3141230Z 
2024-03-15T22:38:17.3141350Z Call Stack (most recent call first):
2024-03-15T22:38:17.3141791Z   /usr/local/share/cmake-3.28/Modules/GoogleTestAddTests.cmake:225 (gtest_discover_tests_impl)

It appears that some combination of google test, ubuntu, and VM/Containers is causes the address sanitizer to trigger an infinite loop which causes the timeout.

See quantumlib/Stim#717 and google/googletest#4491

Changes

• Change from ubuntu-latest to ubuntu-20.04 in github CI runners

Target Environment support

• GitHub CI ubuntu runner

[hellkite500]

Not sure why they are just now causing the CI to fail considering the change has been merged for 7 months, but the pedantic errors are causing failures based on a lot of extra ; throughout the code. I'll trace these down and push that fix as a commit to this PR as well.

[PhilMiller]

Github staff integrated a workaround in their CI runner image: actions/runner-images#9491 (comment)