Allow Python requests to include +gil to require a GIL-enabled interpreter (#16537)

Addresses
https://github.com/astral-sh/uv/pull/16142#issuecomment-3390586957

Nobody seems to have a good idea about how to spell this. "not
free-threaded" would be the most technically correct, but I think "gil"
will be more intuitive.

crates/uv-python/src/interpreter.rs

@@ -173,7 +173,7 @@ impl Interpreter {
             base_executable,
             self.python_major(),
             self.python_minor(),
-            self.variant().suffix(),
+            self.variant().executable_suffix(),
         ) {
             Ok(path) => path,
             Err(err) => {