[GitHub Actions] Make `macos` to lowercase

.github/workflows/swiftpm.yml

@@ -21,7 +21,7 @@ jobs:
       fail-fast: false
       matrix:
         xcode_version: ['11.3.1', '11.7', '12']
-    runs-on: macOS-10.15
+    runs-on: macos-10.15
     env:
       DEVELOPER_DIR: /Applications/Xcode_${{ matrix.xcode_version }}.app
     steps:

.github/workflows/xcodebuild.yml

@@ -25,7 +25,7 @@ jobs:
         xcode_version: ['11.3.1', '11.7', '12']
         xcode_flags: ['-scheme Base32 -enableCodeCoverage YES -parallel-testing-enabled NO test']
         xcode_flags_for_build_only: ['-scheme Base32 build']
-    runs-on: macOS-10.15
+    runs-on: macos-10.15
     env:
       DEVELOPER_DIR: /Applications/Xcode_${{ matrix.xcode_version }}.app
     steps: