Referencing plugin from buildSrc in injected build

I have a source dependency on a project that doesn’t use Gradle. I’m injecting a build using a one-off plugin:

package ....plugins.buildinjection;


import ...SomePlugin

class BuildInjectionPlugin implements Plugin<Settings> {

    @Override
    public void apply(Settings settings) {
        settings.gradle.rootProject {
            project.pluginManager.apply(SomePlugin.class)
        }
    }

}

I include the build for this plugin into the main project:

includeBuild('plugins/build-injection')

Everything works fine like this.

I would like to simplify things, though, by moving the code for this plugin to buildSrc. If I do this, I start getting errors, because buildSrc is not searched by the injected build:

* What went wrong:
Plugin [id: 'build-injection'] was not found in any of the following sources:

- Gradle Core Plugins (not a core plugin, please see https://docs.gradle.org/7.4.2/userguide/standard_plugins.html for available core plugins)
- Included Builds (None of the included builds contain this plugin)
- Plugin Repositories (plugin dependency must include a version number for this source)

Is there a technical reason why buildSrc wouldn’t also be relevant for the injected build?