feat: lake: .nobuild trace file for debugging (#12076)

This PR adds additional debugging information to a run of `lake build
--no-build` via a `.nobuild` trace file. When a build fails due to
needing a rebuild, Lake emits the new expected trace next as `.nobuild`
file next to the build's old `.trace`. The inputs recorded in these
files can then be compared to debug what caused the mismatch.

To help keep the build directory clean, the `.nobuild` trace file is
removed on the next successful build.

src/lake/Lake/Build/Common.lean

@@ -9,7 +9,7 @@ prelude
 public import Lake.Build.Job.Monad
 public import Lake.Config.Monad
 public import Lake.Util.JsonObject
-import Lake.Util.IO
+public import Lake.Util.IO
 import Lake.Build.Target.Fetch
 public import Lake.Build.Actions
 
@@ -304,8 +304,10 @@ and log are saved to `traceFile`, if the build completes without a fatal error
   (depTrace : BuildTrace) (traceFile : FilePath) (build : JobM α)
   (action : JobAction := .build)
 : JobM α := do
+  let noBuildTraceFile := traceFile.addExtension "nobuild"
   if (← getNoBuild) then
     updateAction .build
+    writeBuildTrace noBuildTraceFile depTrace Json.null {}
     error s!"target is out-of-date and needs to be rebuilt"
   else
     updateAction action
@@ -315,6 +317,7 @@ and log are saved to `traceFile`, if the build completes without a fatal error
       let a ← build -- fatal errors will abort here
       let log := (← getLog).takeFrom iniPos
       writeBuildTrace traceFile depTrace (toOutputJson a) log
+      removeFileIfExists noBuildTraceFile
       return a
     finally
       let endTime ← IO.monoMsNow