Skip using Scala nightlies in unit tests when they can't be fetched

[Gedochao]

This should fix the CI failing on main:

scala.build.tests.BuildOptionsTests:
==> X scala.build.tests.BuildOptionsTests.initializationError  0.001s java.lang.UnsupportedOperationException: empty.maxBy
    at scala.collection.IterableOnceOps.maxBy(IterableOnce.scala:1160)
    at scala.collection.IterableOnceOps.maxBy$(IterableOnce.scala:1158)
    at scala.collection.AbstractIterable.maxBy(Iterable.scala:935)
    at scala.build.tests.BuildOptionsTests.<init>(BuildOptionsTests.scala:333)
    at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
    at jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
    at java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:128)
    at jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:347)
    at java.lang.Class.newInstance(Class.java:645)
    at munit.MUnitRunner.MUnitRunner$superArg$1$$anonfun$1(MUnitRunner.scala:32)
    at munit.MUnitRunner.<init>(MUnitRunner.scala:34)
    at munit.MUnitRunner.<init>(MUnitRunner.scala:32)
    at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
    at jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:481)
    at munit.internal.junitinterface.JUnitComputer.getRunner(JUnitComputer.java:75)
    at munit.internal.junitinterface.JUnitComputer$1.runnerForClass(JUnitComputer.java:65)
    ...

https://github.com/VirtusLab/scala-cli/actions/runs/13280636786/job/37078168991#step:8:701