ci: run on ubuntu-20.04

ubuntu-latest is now ubuntu-22.04 instead of 20.04 [1]. Module CI uses
older versions of Tarantool in tests (like 1.10.6 and 2.2) [2]. There is
no such versions in Ubuntu 22.04 repos. So until we decide to reduce our
test matrix, we will use ubuntu-20.04 for tests. Since there is at
least one test pipeline that should use 20.04, we use it everywhere for
consistency.

1. https://github.blog/changelog/2022-11-09-github-actions-ubuntu-latest-workflows-will-use-ubuntu-22-04/
2. tarantool/setup-tarantool#36

Author: DifferentialOrange

.github/workflows/check_on_push.yaml

@@ -9,7 +9,7 @@ jobs:
     if: |
       github.event_name == 'push' ||
       github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@master
 

.github/workflows/push_rockspec.yaml

@@ -12,7 +12,7 @@ env:
 
 jobs:
   push-scm-rockspec:
-    runs-on: [ ubuntu-latest ]
+    runs-on: ubuntu-20.04
     if: github.ref == 'refs/heads/master'
     steps:
       - uses: actions/checkout@master
@@ -23,7 +23,7 @@ jobs:
           files: ${{ env.ROCK_NAME }}-scm-1.rockspec
 
   push-tagged-rockspec:
-    runs-on: [ ubuntu-latest ]
+    runs-on: ubuntu-20.04
     if: startsWith(github.ref, 'refs/tags')
     steps:
       - uses: actions/checkout@master

.github/workflows/test_on_push.yaml

@@ -29,7 +29,9 @@ jobs:
             coveralls: true
             metrics-version: "0.12.0"
       fail-fast: false
-    runs-on: [ubuntu-latest]
+    # Can't install older versions on 22.04,
+    # see https://github.com/tarantool/setup-tarantool/issues/36
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@master
 
@@ -80,7 +82,7 @@ jobs:
         tarantool-version: ["1.10", "2.8"]
         metrics-version: ["0.12.0"]
       fail-fast: false
-    runs-on: [ubuntu-latest]
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@master
 
@@ -113,7 +115,7 @@ jobs:
         bundle_version: [ "1.10.11-0-gf0b0e7ecf-r422", "2.7.3-0-gdddf926c3-r422" ]
         metrics-version: ["", "0.12.0"]
       fail-fast: false
-    runs-on: [ ubuntu-latest ]
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@master