Set target when packaging buildpack (#237)

We're seeing some releases with extra `{os}-{arch}` suffixes on the
`.cnb` files, and duplicated copies of `.cnb` files:

- https://github.com/heroku/buildpacks-procfile/releases/tag/v3.1.2
- https://github.com/heroku/buildpacks-python/releases/tag/v0.11.0
- https://github.com/heroku/buildpacks-python/releases/tag/v0.12.0

This seems to be happening because newer versions of pack with
multi-arch support are detecting these buildpacks as multi-arch
buildpacks, and thus tries to build a `"{os}-{arch}.cnb"` for each of
the targets:
https://github.com/buildpacks/pack/pull/2086/files#diff-f149889a3ad04f6ef0e49a7bc274d34be83c6e9ce07539c4ad4c65561fdc7bbaR184-R185

This PR forces `pack buildpack package` to use a `--target` argument,
like `--target linux/amd64` which causes `pack` to skip a bunch of the
multi-arch concerns (multi-arch concerns are already handled by this
automation) like these suffixes.

.github/workflows/_buildpacks-release.yml

@@ -302,7 +302,8 @@ jobs:
             for target in $(jq --exit-status -c ".targets | .[]" <<< "${buildpack}"); do
               output_dir=$(jq --exit-status -r ".output_dir" <<< "${target}")
               cnb_file=$(jq --exit-status -r ".cnb_file" <<< "${target}")
-              pack buildpack package "$cnb_file" --config "${output_dir}/package.toml" --format file --verbose
+              oci_target=$(jq --exit-status -r ".oci_target" <<< "${target}")
+              pack buildpack package "$cnb_file" --target "${oci_target}" --config "${output_dir}/package.toml" --format file --verbose
             done
           done
 

src/commands/generate_buildpack_matrix/command.rs

@@ -119,6 +119,7 @@ pub(crate) struct TargetInfo {
     os: Option<String>,
     arch: Option<String>,
     rust_triple: Option<String>,
+    oci_target: String,
     cnb_file: String,
     stable_tag: String,
     temporary_tag: String,
@@ -161,6 +162,7 @@ pub(crate) fn read_buildpack_info(
                 Ok(TargetInfo {
                     cnb_file: cnb_file(&buildpack_descriptor.buildpack().id, suffix.as_deref()),
                     os: target.os.clone(),
+                    oci_target: oci_target(target),
                     arch: target.arch.clone(),
                     output_dir: target_output_dir(
                         &buildpack_descriptor.buildpack().id,
@@ -220,6 +222,15 @@ fn cnb_file(buildpack_id: &BuildpackId, suffix: Option<&str>) -> String {
     )
 }
 
+// Returns the OCI target name for a buildpack target. Currently ignores distros.
+fn oci_target(target: &BuildpackTarget) -> String {
+    match (target.os.as_deref(), target.arch.as_deref()) {
+        (Some(os), Some(arch)) => format!("{os}/{arch}"),
+        (Some(os), None) => os.to_string(),
+        (None, _) => String::new(),
+    }
+}
+
 // Returns the target naming suffix for image tags and .cnb files.
 fn target_name(target: &BuildpackTarget) -> String {
     match (target.os.as_deref(), target.arch.as_deref()) {
@@ -374,6 +385,8 @@ mod tests {
             bp_info.targets[1].rust_triple,
             Some("aarch64-unknown-linux-musl".to_string())
         );
+        assert_eq!(bp_info.targets[0].oci_target, "linux/amd64");
+        assert_eq!(bp_info.targets[1].oci_target, "linux/arm64");
         assert_eq!(
             bp_info.targets[0].temporary_tag,
             "docker.io/heroku/buildpack-fakey:_918273_linux-amd64"