The "Update" does not work on mono (Linux)

#### Issue details
MP was never able to do a decent upgrade, some versions appear to do the thing, then fail to restart the new version, some just fail. - like the current release, when checking for beta.

#### Version
1.3.76 build xx 15962

#### Platform
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
N/A

#### Hardware type
N/A

#### Logs
```
NFO MissionPlanner.Controls.ProgressReporterDialogue - RunBackgroundOperation
INFO MissionPlanner.Controls.ProgressReporterDialogue - Focus ctl
INFO MissionPlanner.Controls.ProgressReporterDialogue - in focus invoke
INFO MissionPlanner.Controls.ProgressReporterDialogue - DoWork
INFO MissionPlanner.Utilities.Update - get checksums https://ci.appveyor.com/api/projects/meee1/missionplanner/artifacts/checksums.txt?tag=beta - base https://ci.appveyor.com/api/projects/meee1/missionplanner/artifacts/MissionPlannerBeta.zip?tag=beta
ERROR MissionPlanner.Utilities.Update - Update Failed
System.Net.Http.HttpRequestException: Response status code does not indicate success: 404 (Not Found).
at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode () [0x00046] in <3588d5c507ec48a6b5d13a6d5ae64da2>:0
at System.Net.Http.HttpClient.GetStringAsyncCore (System.Threading.Tasks.Task`1[TResult] getTask) [0x00099] in <3588d5c507ec48a6b5d13a6d5ae64da2>:0
at MissionPlanner.Utilities.Update.CheckMD5 (MissionPlanner.Utilities.IProgressReporterDialogue frmProgressReporter, System.String md5url, System.String baseurl) [0x00043] in <2ab45e170aba4134809e32e48270b306>:0
at MissionPlanner.Utilities.Update.updateCheckMain (MissionPlanner.Utilities.IProgressReporterDialogue frmProgressReporter) [0x00080] in <2ab45e170aba4134809e32e48270b306>:0
CustomMessageBox.Show
CustomMessageBox thread calling ProgressReporterDialogue Background thread
CustomMessageBox thread running invoke Base Thread

```

Start with MissionPlanner.Utilities.Update, especially CheckMD5 and updateCheckMain, and reproduce the beta update on Mono/Linux. Inspect how the checksums.txt and MissionPlannerBeta.zip URLs are built and how the restart is handled; done means the update completes and the new version restarts successfully without the logged 404 failure.

Written by the indexing model from the issue text.