Do not assume that msys is on the C drive.

CMakeLists.txt

@@ -293,13 +293,17 @@ include(GNUInstallDirs)
 
 set(Python_POSTFIX "3")
 if(WIN32)
-  # Set the PYTHON_EXECUTABLE variable explicitly to ensure
-  # that we use the msys2 python, not anything else that may
-  # be floating around too (like on github runners...).
-#  set(Python_EXECUTABLE "/usr/bin/python3")
-#  set(PYTHON_EXECUTABLE "/usr/bin/python3")  
+  # Set a hint for finding Python, so we hopefully get the msys
+  # version. This otherwise breaks on github runners.
+  execute_process(
+    COMMAND cygpath -m /ucrt64/usr/bin/
+    OUTPUT_VARIABLE PYTHON_HINT
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+  )
+  find_package(Python COMPONENTS Interpreter Development HINTS ${PYTHON_HINT})
+else()
+  find_package(Python COMPONENTS Interpreter Development)
 endif()
-find_package(Python COMPONENTS Interpreter Development)
 find_package(pybind11 CONFIG)
 if (NOT pybind11_FOUND)
   message(STATUS "Using included pybind11.")

cmake/functions.cmake

@@ -24,6 +24,6 @@ endmacro()
 # Macro just like `install`, but converting the path from a unix
 # path to a windows path using `cygpath`.
 macro(winstall TMP1 FILE TMP2 DEST)
-  execute_process(COMMAND cygpath -m ${FILE} OUTPUT_VARIABLE WFILE)
+  execute_process(COMMAND cygpath -m ${FILE} OUTPUT_VARIABLE WFILE OUTPUT_STRIP_TRAILING_WHITESPACE)
   install(FILES ${WFILE} DESTINATION ${DEST})
 endmacro()