workflow: pin macos version to 10.15 for now

This is because xcode 11 is not available on newer macos, and xcode 11
is required for building conda packages. Here in zprocess we don't
actually need to do compilation since we're a pure package, but we'll
keep this in line with the standard workflow across all our projects.

.github/workflows/release.yml

@@ -42,10 +42,10 @@ jobs:
           - { os: ubuntu-latest,   python: '3.8',  arch: x64 }
           - { os: ubuntu-latest,   python: '3.7',  arch: x64 }
 
-          - { os: macos-latest,    python: '3.10',  arch: x64 }
-          - { os: macos-latest,    python: '3.9',  arch: x64 }
-          - { os: macos-latest,    python: '3.8',  arch: x64 }
-          - { os: macos-latest,    python: '3.7',  arch: x64 }
+          - { os: macos-10.15,    python: '3.10',  arch: x64 }
+          - { os: macos-10.15,    python: '3.9',  arch: x64 }
+          - { os: macos-10.15,    python: '3.8',  arch: x64 }
+          - { os: macos-10.15,    python: '3.7',  arch: x64 }
 
           - { os: windows-latest,  python: '3.10',  arch: x64 }
           - { os: windows-latest,  python: '3.9',  arch: x64 }