Run CI on Ubuntu 20.04

I believe recent failures in CI (`AddressSanitizer::DEADLYSIGNAL`
repeating ad infinitum before a single test actually runs) are due to
using a Debian image with an ASAN bug that affects systems where ASLR
has a wide range (32 bits), combined with running on Ubuntu-latest which
is now setting that ASLR range in the kernel.

See https://github.com/llvm/llvm-project/commit/fb77ca05ffb4f8e666878f2f6718a9fb4d686839

Switching to 20.04 seems to solve the issue.

Fixes #305

.github/workflows/regular_ci.yml

@@ -33,7 +33,7 @@ jobs:
 
   Debian-bullseye_build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     container:
       image: debian:bullseye
     strategy:
@@ -109,7 +109,7 @@ jobs:
 
   Alpine-latest_build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false # Upload src/igr-tests/*/output/* files in igr-tests
       matrix: