Fix AGENT_COMMIT_OVERRIDE environment variable name (#6530)

In PR #4403 a new environment variable has been introduced to use a
commit hash consistent with the hash of the elastic-agent-core DRA
binary used. The env var name was not initialized and ended up using
"MANIFEST_URL" which is obviously incorrect.

This commit sets the correct name AGENT_COMMIT_OVERRIDE to avoid using
the manifest url as a commit hash

dev-tools/mage/settings.go

@@ -48,7 +48,7 @@ const (
 	//ManifestUrlEnvVar is the name fo the environment variable containing the Manifest URL to be used for packaging agent
 	ManifestUrlEnvVar = "MANIFEST_URL"
 	// AgentCommitHashEnvVar allows to override agent commit hash string during packaging
-	AgentCommitHashEnvVar
+	AgentCommitHashEnvVar = "AGENT_COMMIT_HASH_OVERRIDE"
 
 	// Mapped functions
 	agentPackageVersionMappedFunc    = "agent_package_version"