[ci] Build releases on Ubuntu 20.04

This is the version of Ubuntu we support for OpenTitan (i.e. in its CI
and the dev container).

Building releases for 22.04 binds the binaries to a newer glibc than
Ubuntu 20.04 has access to, so those binaries cannot be run there.

Signed-off-by: James Wainwright <[email protected]>

Author: jwnrt

.github/workflows/create_release.yml

@@ -15,7 +15,7 @@ jobs:
       BUILD_DIR: build
       BRANCH_NAME: ${{ github.event.repository.default_branch }}
       RELEASE_BIN_ARCHIVE: qemu-ot-earlgrey-${{ inputs.release_tag }}-x86_64-unknown-linux-gnu.tar.gz
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
     steps:
       - name: Check out repository