config: use URL method to get Nomad address for plugins

For #944 we fixed the Nomad API package so that it no longer mutated the private
`url` field if previously set, which allowed reusing an `api.Config` object
between clients when a unix domain socket was in use.

However, the autoscaler plugins for Nomad strategy and target don't use the
`api.Config` object we parse directly and instead get a map of string->string
derived from that config so it can be passed over the go-plugin interface. This
mapping did not account for the `Address` field being mutated when unix domain
sockets are in use, so the bug was not actually fixed.

Update the mapping to use the safe `URL()` method on the config, rather than
reading the `Address` field.

Fixes: #955
Ref: hashicorp/nomad#23785

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## UNRELEASED
 
+BUG FIXES:
+* config: Fixed a bug where the Nomad strategy and target plugins would fail to connect to Nomad's Task API socket [[GH-966](https://github.com/hashicorp/nomad-autoscaler/pull/966)]
+
 ## 0.4.5 (August 13, 2024)
 
 IMPROVEMENTS:

sdk/helper/nomad/config.go

@@ -112,7 +112,7 @@ func MergeMapWithAgentConfig(m map[string]string, cfg *api.Config) {
 	}
 
 	if cfg.Address != "" && m[configKeyNomadAddress] == "" {
-		m[configKeyNomadAddress] = cfg.Address
+		m[configKeyNomadAddress] = cfg.URL().String()
 	}
 	if cfg.Region != "" && m[configKeyNomadRegion] == "" {
 		m[configKeyNomadRegion] = cfg.Region