More support for std::filepath in libnixutil

We're not replacing `Path` in exposed definitions in many cases, but
just adding alternatives. This will allow us to "top down" change `Path`
to `std::fileysystem::path`, and then we can remove the `Path`-using
utilities which will become unused.

Also add some test files which we forgot to include in the libutil unit
tests `meson.build`.

Co-Authored-By: siddhantCodes <siddhantk232@gmail.com>

src/libstore/gc.cc

@@ -333,7 +333,7 @@ static std::string quoteRegexChars(const std::string & raw)
 }
 
 #if __linux__
-static void readFileRoots(const char * path, UncheckedRoots & roots)
+static void readFileRoots(const std::filesystem::path & path, UncheckedRoots & roots)
 {
     try {
         roots[readFile(path)].emplace(path);