Fix evalState::rootFS paths' to_string()

This showPath is getting a little too ad hoc, but it works for now.

src/libexpr/eval.cc

@@ -315,6 +315,7 @@ EvalState::EvalState(
 {
     corepkgsFS->setPathDisplay("<nix", ">");
     internalFS->setPathDisplay("«nix-internal»", "");
+    rootFS->setPathDisplay("/", "");
 
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 

src/libutil/source-accessor.cc

@@ -64,7 +64,9 @@ void SourceAccessor::setPathDisplay(std::string displayPrefix, std::string displ
 
 std::string SourceAccessor::showPath(const CanonPath & path)
 {
-    return displayPrefix + (displayPrefix.empty() ? "" : ":") + path.rel() + displaySuffix;
+    return displayPrefix
+        + ((displayPrefix.empty() || displayPrefix.ends_with("/")) ? "" : ":") + path.rel()
+        + displaySuffix;
 }
 
 CanonPath SourceAccessor::resolveSymlinks(