uv step: checking self subcommand exits; fixes #942 (#971)

* uv step: checking self subcommand exits; fixes #942

* uv: fixing return behavior

---------

Co-authored-by: Lucas Parzianello <lucaspar@users.noreply.github.com>

src/steps/generic.rs

@@ -1032,15 +1032,20 @@ pub fn run_uv(ctx: &ExecutionContext) -> Result<()> {
     let uv_exec = require("uv")?;
     print_separator("uv");
 
+    // try uv self --help first - if it succeeds, we call uv self update
+    let result = ctx
+        .run_type()
+        .execute(&uv_exec)
+        .args(["self", "--help"])
+        .output_checked();
+
+    if result.is_ok() {
         ctx.run_type()
             .execute(&uv_exec)
             .args(["self", "update"])
             .status_checked()
             .ok();
-
+    }
-    // ignoring self-update errors, because they are likely due to uv's
-    // installation being managed by another package manager, in which
-    // case another step will handle the update.
 
     ctx.run_type()
         .execute(&uv_exec)