[CI] Pin GHA runner versions (#604)

Author: straight-shoota

.github/workflows/ci.yml

@@ -11,10 +11,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-22.04, macos-12]
         crystal: [latest, nightly]
         include:
-          - os: windows-latest
+          - os: windows-2022
             crystal: nightly
     runs-on: ${{ matrix.os }}
 
@@ -47,13 +47,13 @@ jobs:
           sudo apt-get install fossil
 
       - name: Install Fossil
-        if: ${{ matrix.os == 'macos-latest' }}
+        if: ${{ matrix.os == 'macos-12' }}
         run: |
           brew update
           brew install fossil
 
       - name: Install Fossil
-        if: ${{ matrix.os == 'windows-latest' }}
+        if: ${{ matrix.os == 'windows-2022' }}
         run: choco install fossil
 
       - name: Download source
@@ -69,7 +69,7 @@ jobs:
         run: crystal tool format --check src spec
 
   docs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Download source
         uses: actions/checkout@v2