Move user binstubs to its own layer

At runtime, the alphabetical order of the layer name determines the order it is loaded. At build time, the order that the `env` variable is modified determines the order.

At both build and runtime we want the bin stubs to come first on the PATH when executing user defined code. This was already working for runtime, but wasn't for build time as the "gems" layer was being prepended to the path after the "venv" layer (because the `venv` layer was being defined first, last definition wins).

I originally tried to fix this by defining the PATH inside of the "gems" layer along with the gems path but ran into heroku/libcnb.rs#899. The libcnb.rs project loads the user defined PATH modification last, but I'm unclear if that's spec defined behavior or not https://github.com/buildpacks/spec/blob/main/buildpack.md#layer-paths.

buildpacks/ruby/src/main.rs

@@ -11,9 +11,11 @@ use layers::{
 use libcnb::build::{BuildContext, BuildResult, BuildResultBuilder};
 use libcnb::data::build_plan::BuildPlanBuilder;
 use libcnb::data::launch::LaunchBuilder;
+use libcnb::data::layer_name;
 use libcnb::detect::{DetectContext, DetectResult, DetectResultBuilder};
 use libcnb::generic::{GenericMetadata, GenericPlatform};
-use libcnb::layer_env::Scope;
+use libcnb::layer::UncachedLayerDefinition;
+use libcnb::layer_env::{LayerEnv, ModificationBehavior, Scope};
 use libcnb::Platform;
 use libcnb::{buildpack_main, Buildpack};
 use std::io::stdout;
@@ -220,6 +222,28 @@ impl Buildpack for RubyBuildpack {
             (bullet.done(), layer_env.apply(Scope::Build, &env))
         };
 
+        env = {
+            let user_binstubs = context.uncached_layer(
+                layer_name!("user_binstubs"),
+                UncachedLayerDefinition {
+                    build: true,
+                    launch: true,
+                },
+            )?;
+            user_binstubs.write_env(
+                LayerEnv::new()
+                    .chainable_insert(Scope::All, ModificationBehavior::Delimiter, "PATH", ":")
+                    .chainable_insert(
+                        Scope::All,
+                        ModificationBehavior::Prepend,
+                        "PATH",
+                        context.app_dir.join("bin"),
+                    ),
+            )?;
+
+            user_binstubs.read_env()?.apply(Scope::Build, &env)
+        };
+
         // ## Detect gems
         let (mut build_output, gem_list, default_process) = {
             let bullet = build_output.bullet("Default process detection");

buildpacks/ruby/src/steps/default_env.rs

@@ -38,20 +38,12 @@ pub(crate) fn default_env(
         .to_string();
 
     let layer_ref = context.uncached_layer(
-        layer_name!("venv"),
+        layer_name!("env_defaults"),
         UncachedLayerDefinition {
             build: true,
             launch: true,
         },
     )?;
-    let update_env = LayerEnv::new()
-        .chainable_insert(Scope::All, ModificationBehavior::Delimiter, "PATH", ":")
-        .chainable_insert(
-            Scope::All,
-            ModificationBehavior::Prepend,
-            "PATH",
-            context.app_dir.join("bin"),
-        );
     let env = layer_ref
         .write_env({
             [
@@ -65,7 +57,7 @@ pub(crate) fn default_env(
                 ("DISABLE_SPRING", "1"),
             ]
             .iter()
-            .fold(update_env, |layer_env, (name, value)| {
+            .fold(LayerEnv::new(), |layer_env, (name, value)| {
                 layer_env.chainable_insert(Scope::All, ModificationBehavior::Default, name, value)
             })
         })