libutil: don't save cwd fd, use path instead

Saving the cwd fd didn't actually work well -- prior to this commit, the
following would happen:

    : ~/w/vc/nix ; doas outputs/out/bin/nix --experimental-features 'nix-command flakes' run nixpkgs#coreutils -- --coreutils-prog=pwd
    pwd: couldn't find directory entry in ‘../../../..’ with matching i-node
    : ~/w/vc/nix ; doas outputs/out/bin/nix --experimental-features 'nix-command flakes' develop -c pwd
    pwd: couldn't find directory entry in ‘../../../..’ with matching i-node

src/libutil/util.cc

@@ -1709,13 +1709,11 @@ void restoreMountNamespace()
 {
 #if __linux__
     try {
-        AutoCloseFD fdSavedCwd = open("/proc/self/cwd", O_RDONLY);
-        if (!fdSavedCwd) {
-            throw SysError("saving cwd");
-        }
+        auto savedCwd = absPath(".");
+
         if (fdSavedMountNamespace && setns(fdSavedMountNamespace.get(), CLONE_NEWNS) == -1)
             throw SysError("restoring parent mount namespace");
-        if (fdSavedCwd && fchdir(fdSavedCwd.get()) == -1) {
+        if (chdir(savedCwd.c_str()) == -1) {
             throw SysError("restoring cwd");
         }
     } catch (Error & e) {