Improve SourceAccessor path display

Backported from lazy-trees. This allows SourceAccessors to show the
origin of the accessor. E.g. we now get

  copying '«git+https://github.com/blender/blender.git?ref=refs/heads/main&rev=4edc1389337dd3679ff66969c332d2aff52e1992»/' to the store

instead of

  copying '/' to the store

src/libfetchers/fs-input-accessor.cc

@@ -18,6 +18,7 @@ struct FSInputAccessorImpl : FSInputAccessor, PosixSourceAccessor
         , allowedPaths(std::move(allowedPaths))
         , makeNotAllowedError(std::move(makeNotAllowedError))
     {
+        displayPrefix = root.isRoot() ? "" : root.abs();
     }
 
     void readFile(