fix: explicitly run default environment (prefix-dev#2273)

```
pixi run -e default xxx
```
Would not result in an explicit env run but would act like an auto
detected run, still prompting the environment selector.

src/cli/run.rs

@@ -59,7 +59,7 @@ pub async fn execute(args: Args) -> miette::Result<()> {
     let environment = project.environment_from_name_or_env_var(args.environment.clone())?;
 
     // Find the environment to run the task in, if any were specified.
-    let explicit_environment = if environment.is_default() {
+    let explicit_environment = if args.environment.is_none() {
         None
     } else {
         Some(environment.clone())