Use os.copyfile() instead of the '{COPY}' command

{COPY} doesn't work on windows.

Diffs=
ea1c83d02 Use os.copyfile() instead of the '{COPY}' command
diff --git a/.rive_head b/.rive_head
index b2df691..b8230be 100644
--- a/.rive_head
+++ b/.rive_head
@@ -1 +1 @@
-5c03e164093d308c72626d079201479a8a325256
+ea1c83d025d5991567973478df416e64f6888ad4
diff --git a/dependencies/premake5_libpng.lua b/dependencies/premake5_libpng.lua
index 9b567c3..188dc91 100644
--- a/dependencies/premake5_libpng.lua
+++ b/dependencies/premake5_libpng.lua
@@ -13,7 +13,7 @@
         '-fno-exceptions',
         '-fno-rtti'
     }
-    prebuildcommands {'{COPY} ' .. libpng .. '/scripts/pnglibconf.h.prebuilt ' .. libpng .. '/pnglibconf.h'}
+    os.copyfile(libpng .. '/scripts/pnglibconf.h.prebuilt', libpng .. '/pnglibconf.h')
     includedirs {
         './',
         libpng