Use "python3" in make_viewer_skia.sh (instead of "python")

"python" doesn't exist on all the hosted runners, so it was a game of runner-roulette whether the runtime_test_macos job will pass.

Diffs=
63b6a13d3 Use "python3" in make_viewer_skia.sh (instead of "python") (#7597)

Co-authored-by: Chris Dalton <99840794+csmartdalton@users.noreply.github.com>
diff --git a/.rive_head b/.rive_head
index 61466a3..2db75dc 100644
--- a/.rive_head
+++ b/.rive_head
@@ -1 +1 @@
-8a67331a40847538f893acddfcc57e249e21ec81
+63b6a13d3a2adc1c555747fcd76234360bad7a1d
diff --git a/dependencies/macosx/make_viewer_skia.sh b/dependencies/macosx/make_viewer_skia.sh
index f029436..507781e 100755
--- a/dependencies/macosx/make_viewer_skia.sh
+++ b/dependencies/macosx/make_viewer_skia.sh
@@ -17,7 +17,7 @@
     cd skia
 fi
 
-python tools/git-sync-deps
+python3 tools/git-sync-deps
 
 CONFIG=debug
 RENDERER=