macos-13 is the latest, free x86_64 target

.github/workflows/build.yml

@@ -1,7 +1,7 @@
 # Build, test and upload executables to GitHub
 # depends on pre-installed software; see image definitions:
 #  - https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2404-Readme.md
-#  - https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md
+#  - https://github.com/actions/runner-images/blob/main/images/macos/macos-13-Readme.md
 # We use these tools from the runner images: git, gcc/XCode/MSVC, swig, Tcl/Tk on MacOS, pkg-config
 
 # TODO: not building svs_viewer ('Cannot find GL, GLU, not building svs_viewer' and 'Cannot find opengl32, glu32, not building svs_viewer')
@@ -31,7 +31,7 @@ jobs:
           # Use v24 beta for now to get newer SWIG; TODO: change back to ubuntu-latest once it points to v24
           ubuntu-24.04,
           # latest available X86_64 target
-          macos-15,
+          macos-13,
           # latest is ARM
           macos-latest,
         ]
@@ -286,7 +286,7 @@ jobs:
             # 20.04 to preserve compatibility with testing stage
             ubuntu-20.04,
             # latest available X86_64 target
-            macos-15,
+            macos-13,
             # latest is ARM
             macos-latest,
 
@@ -372,7 +372,7 @@ jobs:
           # linux target which supports all python versions we want to install for
           - ubuntu-20.04
             # latest available X86_64 target
-          - macos-15
+          - macos-13
             # latest is ARM
           - macos-latest
           - windows-latest