Fix errors building Ruby 3.1 on macos 15

by pinning to macos-14. I believe this problem started when Github
upgraded to macOS 15:

> The macos-latest label will migrate to macOS 15 beginning August 4, 2025

The error when system libraries are disabled:

    ./libtool: line 1871: /Applications/Xcode_15.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ar: No such file or directory

The error when system libraries are enabled:

    make: /Applications/Xcode_15.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ar: No such file or directory

(cherry picked from commit 1dac08d)

Author: flavorjones

.github/workflows/ci.yml

@@ -271,7 +271,9 @@ jobs:
       matrix:
         sys: ["enable", "disable"]
         ruby: ${{ fromJSON(needs.ruby_versions.outputs.setup_ruby) }}
-    runs-on: macos-latest
+        include:
+          - { ruby: "3.1", os: macos-14 }
+    runs-on: ${{ matrix.os || 'macos-latest' }}
     steps:
       - uses: actions/checkout@v4
         with:
@@ -526,7 +528,9 @@ jobs:
       matrix:
         sys: ["enable", "disable"]
         ruby: ${{ fromJSON(needs.ruby_versions.outputs.setup_ruby) }}
-    runs-on: macos-latest
+        include:
+          - { ruby: "3.1", os: macos-14 }
+    runs-on: ${{ matrix.os || 'macos-latest' }}
     steps:
       - uses: actions/checkout@v4
         with: