[FIX] Don't allow SSH authentication without ssh executable

- Follow up of #4819
- When no `ssh` executable is present, disable the UI and backend bits
that allow the creation of push mirrors that use SSH authentication. As
this feature requires the usage of the `ssh` binary.
- Integration test added.

routers/web/repo/setting/setting.go

@@ -92,6 +92,7 @@ func SettingsCtxData(ctx *context.Context) {
 		return
 	}
 	ctx.Data["PushMirrors"] = pushMirrors
+	ctx.Data["CanUseSSHMirroring"] = git.HasSSHExecutable
 }
 
 // Units show a repositorys unit settings page
@@ -643,6 +644,11 @@ func SettingsPost(ctx *context.Context) {
 			return
 		}
 
+		if form.PushMirrorUseSSH && !git.HasSSHExecutable {
+			ctx.RenderWithErr(ctx.Tr("repo.mirror_use_ssh.not_available"), tplSettingsOptions, &form)
+			return
+		}
+
 		address, err := forms.ParseRemoteAddr(form.PushMirrorAddress, form.PushMirrorUsername, form.PushMirrorPassword)
 		if err == nil {
 			err = migrations.IsMigrateURLAllowed(address, ctx.Doer)