Switch Linux runner to ubuntu-20.04 to unbreak CI

Earlier today CI started to fail on Linux with a crash during the
jit mode. Turns out ubuntu-latest switched from ubuntu-20.04 to
ubuntu-22.04. Let's switch back to ubuntu-20.04 for now to unbreak
CI until I figured out the root cause.

cc rust-lang#1303

.github/workflows/main.yml

@@ -29,7 +29,7 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-20.04 # FIXME switch to ubuntu-22.04 once #1303 is fixed
             env:
               TARGET_TRIPLE: x86_64-unknown-linux-gnu
           - os: macos-latest