fix(cache): build job URLs on the address its runner registered (#1153)

The cache server built every URL it hands a job from its own listen address, so jobs whose runner reaches it through a reverse proxy were sent to the internal one. This covered the v1 `archiveLocation`, the v2 signed cache URLs and `ACTIONS_RESULTS_URL`.

Runners now register the address their jobs reach the server at, next to the instance URL they already send. The cache-server needs no configuration of its own, and runners that reach it differently each get their own correct address.

Closes https://gitea.com/gitea/runner/issues/1152

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1153
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: Max P. <mail@0xMax42.io>

internal/pkg/config/config.example.yaml

@@ -137,6 +137,7 @@ cache:
   #port: 0
   # URL of a shared `gitea-runner cache-server` to use instead of starting a local one.
   # Set on every runner that should share a cache pool. A trailing slash is optional.
+  # Jobs reach the server at this URL too, so set it to the reverse proxy when one fronts the server.
   # Example: "http://cache-host:8088/"
   # Requires external_secret (below) to match the value on the cache-server.
   #external_server: ""