Fix orbit active GUI session detection to start Fleet Desktop and key escrowing on Linux (#39777)

Resolves #36024 and #34501.

Main change is about stop using the first user in the `users` command
output [*] and instead use `loginctl` commands to pick the correct
current active GUI user.

[*] `users` was returning empty on some new distributions, and in
multi-sessions we were always picking the first one (even if it wasn't
active).

- [X] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.

## Testing

- [x] QA'd all new/changed functionality manually

## fleetd/orbit/Fleet Desktop

- [x] Verified compatibility with the latest released version of Fleet
(see [Must
rule](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/workflows/fleetd-development-and-release-strategy.md))
- [x] If the change applies to only one platform, confirmed that
`runtime.GOOS` is used as needed to isolate changes
- [x] Verified that fleetd runs on macOS, Linux and Windows
- [x] Verified auto-update works from the released version of component
to the new version (see [tools/tuf/test](../tools/tuf/test/README.md))


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

* **Bug Fixes**
* Fixed Fleet Desktop startup to correctly detect and use the active GUI
session on Linux systems.
* Improved GUI user detection for dialog prompts, ensuring system
dialogs run in the proper user context.

* **Improvements**
* Enhanced error reporting and logging clarity for GUI session detection
failures.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

orbit/pkg/execuser/execuser.go

@@ -3,7 +3,6 @@
 package execuser
 
 import (
-	"io"
 	"time"
 )
 
@@ -70,11 +69,3 @@ func RunWithOutput(path string, opts ...Option) (output []byte, exitCode int, er
 	}
 	return runWithOutput(path, o)
 }
-
-func RunWithStdin(path string, opts ...Option) (io.WriteCloser, error) {
-	var o eopts
-	for _, fn := range opts {
-		fn(&o)
-	}
-	return runWithStdin(path, o)
-}