ci: only copy python.exe to python3.exe if the latter does not exist

We're copying the binary to make sure we can call python3.exe around,
but it seems like the base image of GitHub Actions changed, copying the
file before we do so.

This commit changes the CI script to only copy the file if it doesn't
already exist.

src/ci/scripts/install-msys2.sh

@@ -24,7 +24,9 @@ if isWindows; then
     # baked in which break LLVM's build system one way or another, so let's use the
     # native version which keeps everything as native as possible.
     python_home="/c/hostedtoolcache/windows/Python/${native_python_version}/x64"
-    cp "${python_home}/python.exe" "${python_home}/python3.exe"
+    if ! [[ -f "${python_home}/python3.exe" ]]; then
+        cp "${python_home}/python.exe" "${python_home}/python3.exe"
+    fi
     ciCommandAddPath "C:\\hostedtoolcache\\windows\\Python\\${native_python_version}\\x64"
     ciCommandAddPath "C:\\hostedtoolcache\\windows\\Python\\${native_python_version}\\x64\\Scripts"
 fi