.github/workflows/config.yml: debian-unstable: run-on ubuntu:20.04

```
+ docker run --rm --platform linux/amd64 -v /home/runner/work/EusLisp/EusLisp:/ws/euslisp -e ARCH=LinuxARM -t arm32v7/debian:unstable-slim bash -c '      set -x; set -e;      apt update -qq;      apt install -y -qq make gcc libgl-dev libglu1-mesa-dev libjpeg-dev libpng-dev libpq-dev libx11-dev libxext-dev;      CFLAGS='\''-Werror=implicit-function-declaration'\'' ARCH=LinuxARM EUSDIR=/ws/euslisp make -C /ws/euslisp/lisp/ -f Makefile.LinuxARM eus0;      CC='\''gcc -Werror'\'' ARCHDIR=LinuxARM EUSDIR=/ws/euslisp make -C /ws/euslisp/lisp/image/jpeg;      exit 0'
Unable to find image 'arm32v7/debian:unstable-slim' locally
unstable-slim: Pulling from arm32v7/debian
docker: no matching manifest for linux/amd64 in the manifest list entries.
```

.github/workflows/config.yml

@@ -107,7 +107,7 @@ jobs:
           - DOCKER_IMAGE: riscv64/debian:unstable-slim
             ARCH: Linux
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu:20.04
     timeout-minutes: 60
 
     name: debian-unstale