build: explicitly pass the path to python2,python3

Explicitly pass the path to the python interpreter on macOS by using
`xcrun` to find the tools.

Author: compnerd

utils/build-script-impl

@@ -481,6 +481,10 @@ function set_build_options_for_host() {
         appletvsimulator-*  | \
         watchos-*           | \
         watchsimulator-*)
+            swift_cmake_options+=(
+              -DPython2_EXECUTABLE=$(xcrun -f python2.7)
+              -DPython3_EXECUTABLE=$(xcrun -f python3)
+            )
             case ${host} in
                 macosx-x86_64)
                     SWIFT_HOST_TRIPLE="x86_64-apple-macosx${DARWIN_DEPLOYMENT_VERSION_OSX}"