mantle/kolet: Use PathChanged instead of PathExists

The previous code (changed in 4f4dfe6) copied to a partial file
and then renamed which meant PathExists would trigger as soon
as the file path exists, which was too soon because the file is
still copying.

PathChanged seems to work for our purposes here since it will
trigger when the file is closed after it is done writing.

docs/kola/external-tests.md

@@ -138,7 +138,8 @@ In the following Fedora CoreOS config example, the Ignition config includes a
 path unit and a service unit. The path unit ensures that the httpd service runs
 automatically once the Kolet binary is copied to the system. Note that the path
 unit has a `Before=` dependency on `kola-runext.service` to ensure that the
-server is brought up before the test is run.
+server is brought up before the test is run. Note also that `PathChanged` will
+trigger once the `kolet` binary has finished copying to the system.
 An HTTP server will be started at `localhost` and serve the files in `/var/www/`.
 Your test can then do e.g. `curl localhost/hello_world.txt`.
 
@@ -154,7 +155,7 @@ systemd:
         [Unit]
         Before=kola-runext.service
         [Path]
-        PathExists=/usr/local/bin/kolet
+        PathChanged=/usr/local/bin/kolet
         [Install]
         WantedBy=kola-runext.service
     - name: kolet-httpd.service

mantle/kola/tests/upgrade/basic.go

@@ -80,7 +80,7 @@ func init() {
       {
         "name": "kolet-httpd.path",
         "enabled": true,
-        "contents": "[Path]\nPathExists=/usr/local/bin/kolet\n[Install]\nWantedBy=multi-user.target"
+        "contents": "[Path]\nPathChanged=/usr/local/bin/kolet\n[Install]\nWantedBy=multi-user.target"
       },
       {
         "name": "kolet-httpd.service",