YAML-header defined `mvnDeps` cause a `mill --meta-build 1 show mvnDeps` to fail

[lefou]

Reproducer:

build.mill:

//| mill-version: 1.0.3
//| mvnDeps:
//| - com.lihaoyi::scalatags:0.12.0

> mill --meta-level 1 show mvnDeps
[1/1, 1 failed] ============================== show mvnDeps ============================== 1s
1 tasks failed
show java.nio.file.NoSuchFileException: /tmp/mill-5733/out/mill-build/mvnDeps.json
    java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
    java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:106)
    java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
    java.base/sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:261)
    java.base/java.nio.file.Files.newByteChannel(Files.java:380)
    java.base/java.nio.file.Files.newByteChannel(Files.java:432)
    java.base/java.nio.file.spi.FileSystemProvider.newInputStream(FileSystemProvider.java:420)
    java.base/java.nio.file.Files.newInputStream(Files.java:160)
    ujson.Readable$$anon$1.transform(Readable.scala:26)
    ujson.Readable$.transform(Readable.scala:15)
    ujson.Readable$.transform(Readable.scala:14)
    upickle.core.BufferedValue$.maybeSortKeysTransform(BufferedValue.scala:76)
    ujson.package$.transform(package.scala:8)
    ujson.package$.read$$anonfun$1(package.scala:15)
    upickle.core.TraceVisitor$.withTrace(TraceVisitor.scala:18)
    ujson.package$.read(package.scala:15)
    mill.util.MainModule$.$anonfun$9(MainModule.scala:357)
    scala.collection.immutable.List.map(List.scala:247)
    scala.collection.immutable.List.map(List.scala:79)
    mill.util.MainModule$.show0$$anonfun$1(MainModule.scala:353)
    mill.api.daemon.Result$Success.flatMap(Result.scala:30)
    mill.util.MainModule$.mill$util$MainModule$$$show0(MainModule.scala:347)
    mill.util.MainModule.show$$anonfun$1(MainModule.scala:127)
    mill.api.Task$Named.evaluate(Task.scala:338)
    mill.api.Task$Named.evaluate$(Task.scala:323)
    mill.api.Task$Command.evaluate(Task.scala:410)

[lefou]

Interestingly, while the error looks pretty severe, it doesn't seem to have any negative impact on downstream processing. In fact, it seem to have almost no impact at all, except the mvnDeps task result can't be seen via show. And there is also no out/mill-build/mvnDeps.json file, of course.

[lefou]

My assumption is, that this is a side-effect of how we allow the YAML header contribute to the meta-build in a generic way.