Fix snapshot source URI override, add test.

internal/pkg/agent/application/upgrade/artifact/download/snapshot/downloader.go

@@ -90,7 +90,7 @@ func snapshotConfig(config *artifact.Config, versionOverride *agtversion.ParsedS
 func snapshotURI(versionOverride *agtversion.ParsedSemVer, config *artifact.Config) (string, error) {
 	// Respect a non-default source URI even if the version is a snapshot.
 	if config.SourceURI != artifact.DefaultSourceURI {
-		return "", fmt.Errorf("skip snapshot download due to non-default sourceURI %s", config.SourceURI)
+		return config.SourceURI, nil
 	}
 
 	// snapshot downloader is used also by the 'localremote' impl in case of agent currently running off a snapshot build:

internal/pkg/agent/application/upgrade/artifact/download/snapshot/downloader_test.go

@@ -0,0 +1,27 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package snapshot
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/elastic/elastic-agent/internal/pkg/agent/application/upgrade/artifact"
+	"github.com/elastic/elastic-agent/pkg/version"
+)
+
+func TestNonDefaultSourceURI(t *testing.T) {
+	version, err := version.ParseVersion("8.12.0-SNAPSHOT")
+	require.NoError(t, err)
+
+	config := artifact.Config{
+		SourceURI: "localhost:1234",
+	}
+	sourceURI, err := snapshotURI(version, &config)
+	require.NoError(t, err)
+	require.Equal(t, config.SourceURI, sourceURI)
+
+}