Merge branch 'jx/ci-ubuntu-fix'

Adjust the GitHub CI to newer ubuntu release.

* jx/ci-ubuntu-fix:
  ci: install python on ubuntu
  ci: use the same version of p4 on both Linux and macOS
  ci: remove the pipe after "p4 -V" to catch errors
  github-actions: run gcc-8 on ubuntu-20.04 image

.github/workflows/main.yml

@@ -227,17 +227,15 @@ jobs:
             pool: ubuntu-latest
           - jobname: linux-sha256
             cc: clang
-            os: ubuntu
             pool: ubuntu-latest
           - jobname: linux-gcc
             cc: gcc
             cc_package: gcc-8
-            pool: ubuntu-latest
+            pool: ubuntu-20.04
           - jobname: linux-TEST-vars
             cc: gcc
-            os: ubuntu
             cc_package: gcc-8
-            pool: ubuntu-latest
+            pool: ubuntu-20.04
           - jobname: osx-clang
             cc: clang
             pool: macos-latest
@@ -288,7 +286,6 @@ jobs:
         - jobname: linux-musl
           image: alpine
         - jobname: linux32
-          os: ubuntu32
           image: daald/ubuntu32:xenial
         - jobname: pedantic
           image: fedora