Can't set githubWorkflowScalaVersions := Nil

When setting

githubWorkflowScalaVersions := Nil

the build fails to load with the following error:

java.lang.RuntimeException: Bug generating artifact download steps
	at scala.sys.package$.error(package.scala:30)
	at org.typelevel.sbt.gha.GenerativePlugin$.$anonfun$buildSettings$7(GenerativePlugin.scala:753)
	at scala.collection.immutable.List.map(List.scala:293)
	at org.typelevel.sbt.gha.GenerativePlugin$.$anonfun$buildSettings$5(GenerativePlugin.scala:751)
	at scala.Function1.$anonfun$compose$1(Function1.scala:49)
	at sbt.internal.util.EvaluateSettings$MixedNode.evaluate0(INode.scala:229)
	at sbt.internal.util.EvaluateSettings$INode.evaluate(INode.scala:171)
	at sbt.internal.util.EvaluateSettings.$anonfun$submitEvaluate$1(INode.scala:88)
	at sbt.internal.util.EvaluateSettings.sbt$internal$util$EvaluateSettings$$run0(INode.scala:100)
	at sbt.internal.util.EvaluateSettings$$anon$3.run(INode.scala:95)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:829)
[error] Bug generating artifact download steps
[error] Use 'last' for the full log.

Motivation

In my case, I want to set

githubWorkflowScalaVersions := Nil
githubWorkflowBuildSbtStepPreamble := Nil

in an sbt-projectmatrix driven project, because there are subprojects for each Scala version already, and so there's no need to use sbt's built-in cross building support.

As a workaround, I've set:

githubWorkflowScalaVersions := Seq("per-project-matrix")
githubWorkflowBuildSbtStepPreamble := Nil

but that's a little goofy and not setting a Scala matrix axis would be preferred.

Start in GenerativePlugin.scala around lines 751-753, where artifact download steps are generated, and reproduce the failure with githubWorkflowScalaVersions := Nil. Trace how an empty Scala version list is handled alongside githubWorkflowBuildSbtStepPreamble := Nil. Done means the build loads successfully without a Scala matrix axis and the existing workaround is no longer needed.