(maint) Pin Ruby versions in GHA workflows

Author: luchihoratiu

.github/workflows/checks.yaml

@@ -13,9 +13,9 @@ jobs:
     strategy:
       matrix:
         cfg:
-          - {check: rubocop, os: ubuntu-18.04, ruby: 2.5}
-          - {check: commits, os: ubuntu-18.04, ruby: 2.5}
-          - {check: warnings, os: ubuntu-18.04, ruby: 2.5}
+          - {check: rubocop,  os: ubuntu-18.04, ruby: '2.5'}
+          - {check: commits,  os: ubuntu-18.04, ruby: '2.5'}
+          - {check: warnings, os: ubuntu-18.04, ruby: '2.5'}
 
     runs-on: ${{ matrix.cfg.os }}
     steps:

.github/workflows/rspec_tests.yaml

@@ -13,15 +13,15 @@ jobs:
     strategy:
       matrix:
         cfg:
-          - {os: ubuntu-18.04, ruby: 2.5}
-          - {os: ubuntu-18.04, ruby: 2.6}
-          - {os: ubuntu-18.04, ruby: 2.7}
-          - {os: ubuntu-18.04, ruby: 3.0}
-          - {os: ubuntu-18.04, ruby: jruby-9.2.17.0}
-          - {os: windows-2019, ruby: 2.5}
-          - {os: windows-2019, ruby: 2.6}
-          - {os: windows-2019, ruby: 2.7}
-          - {os: windows-2019, ruby: 3.0}
+          - {os: ubuntu-18.04, ruby: '2.5'}
+          - {os: ubuntu-18.04, ruby: '2.6'}
+          - {os: ubuntu-18.04, ruby: '2.7'}
+          - {os: ubuntu-18.04, ruby: '3.0'}
+          - {os: ubuntu-18.04, ruby: 'jruby-9.2.17.0'}
+          - {os: windows-2019, ruby: '2.5'}
+          - {os: windows-2019, ruby: '2.6'}
+          - {os: windows-2019, ruby: '2.7'}
+          - {os: windows-2019, ruby: '3.0'}
 
     runs-on: ${{ matrix.cfg.os }}
     steps: