Use an ephemeral environment for uv run --with invocations (#14447)

This PR creates separation between the `--with` environment and the
environment we actually run in, which in turn solves issues like
https://github.com/astral-sh/uv/issues/12889 whereby two invocations
share the same `--with` environment, causing them to collide by way of
sharing an overlay.

Closes https://github.com/astral-sh/uv/issues/7643.

crates/uv-python/src/environment.rs

@@ -174,7 +174,7 @@ impl PythonEnvironment {
     /// N.B. This function also works for system Python environments and users depend on this.
     pub fn from_root(root: impl AsRef<Path>, cache: &Cache) -> Result<Self, Error> {
         debug!(
-            "Checking for Python environment at `{}`",
+            "Checking for Python environment at: `{}`",
             root.as_ref().user_display()
         );
         match root.as_ref().try_exists() {