plume/release: move AMI publishing out of modifyReleaseMetadataIndex()

It seems odd to make the AMIs public in the function focused on updating
the release index. Let's do the AMI publicizing directly from
`runFcosRelease` so we can make `modifyReleaseMetadataIndex` truly only
about the release index.

(cherry picked from commit 7cb7297)

mantle/cmd/plume/release.go

@@ -96,7 +96,9 @@ func runFcosRelease(cmd *cobra.Command, args []string) {
 
 	api := getAWSApi()
 	doS3(api)
-	modifyReleaseMetadataIndex(api)
+	rel := getReleaseMetadata(api)
+	makeReleaseAMIsPublic(rel)
+	modifyReleaseMetadataIndex(api, rel)
 }
 
 func getAWSApi() *aws.API {
@@ -182,7 +184,7 @@ func makeReleaseAMIsPublic(rel release.Release) {
 	}
 }
 
-func modifyReleaseMetadataIndex(api *aws.API) {
+func modifyReleaseMetadataIndex(api *aws.API, rel release.Release) {
 	// Note we use S3 directly here instead of
 	// FetchAndParseCanonicalReleaseIndex(), since that one uses the
 	// CloudFronted URL and we need to be sure we're operating on the latest
@@ -225,8 +227,6 @@ func modifyReleaseMetadataIndex(api *aws.API) {
 		plog.Fatalf("creating metadata url: %v", err)
 	}
 
-	rel := getReleaseMetadata(api)
-
 	var commits []release.IndexReleaseCommit
 	for arch, vals := range rel.Architectures {
 		commits = append(commits, release.IndexReleaseCommit{
@@ -263,8 +263,6 @@ func modifyReleaseMetadataIndex(api *aws.API) {
 		}
 	}
 
-	makeReleaseAMIsPublic(rel)
-
 	releaseIdx.Releases = append(releaseIdx.Releases, newIdxRelease)
 
 	releaseIdx.Metadata.LastModified = time.Now().UTC().Format("2006-01-02T15:04:05Z")