Nightly Dotty workflow of 2021-12-01 failed

[github-actions]

See https://github.com/lampepfl/dotty/actions/runs/1523946606

[griggt]

In the nightly_documentation job:

Error:  java.lang.IllegalArgumentException: requirement failed: Source file '/__w/dotty/dotty/scaladoc-js/markdown/resources/code-snippets.css' does not exist.
Error:  	at scala.Predef$.require(Predef.scala:281)
Error:  	at sbt.io.IO$.copyFile(IO.scala:895)
Error:  	at sbt.io.IO$.copyFile(IO.scala:885)
Error:  	at sbt.io.IO$.copyFile(IO.scala:877)
Error:  	at Build$.$anonfun$scaladoc$47(Build.scala:1451)
Error:  	at scala.collection.immutable.List.map(List.scala:293)
Error:  	at Build$.$anonfun$scaladoc$46(Build.scala:1448)
Error:  	at scala.Function1.$anonfun$compose$1(Function1.scala:49)
Error:  	at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
Error:  	at sbt.std.Transform$$anon$4.work(Transform.scala:68)
Error:  	at sbt.Execute.$anonfun$submit$2(Execute.scala:282)
Error:  	at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
Error:  	at sbt.Execute.work(Execute.scala:291)
Error:  	at sbt.Execute.$anonfun$submit$1(Execute.scala:282)
Error:  	at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
Error:  	at sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
Error:  	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
Error:  	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
Error:  	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
Error:  	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
Error:  	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
Error:  	at java.lang.Thread.run(Thread.java:748)
Error:  (scaladoc / renderScaladocScalajsToFile) java.lang.IllegalArgumentException: requirement failed: Source file '/__w/dotty/dotty/scaladoc-js/markdown/resources/code-snippets.css' does not exist.

[griggt]

Seems to be caused by #13954