home: direnv: lib: don't erase pre-existing venv

Turns out `uv venv` isn't idempotent, it removes the existing virtual
environment by default.

Thankfully, there's a flag to fix it.

modules/home/direnv/lib/python.sh

@@ -46,7 +46,7 @@ layout_uv() {
     fi
 
     # create venv if it doesn't exist
-    uv venv -q
+    uv venv -q --allow-existing
 
     export VIRTUAL_ENV
     export UV_ACTIVE=1