Fix nix upgrade-nix profile search

Commit cfe66db updated `nix upgrade-nix` to use
`ExecutablePath::load().find`, which broke the logic for finding the
profile associated with the nix executable. The error looks something
like:

```
$ sudo -i nix upgrade-nix --debug
found Nix in '"/nix/store/46p1z0w9ad605kky62dr53z4h24k2a5r-nix-2.25.2/bin/nix"'
found profile '/nix/store/46p1z0w9ad605kky62dr53z4h24k2a5r-nix-2.25.2/bin'
error: directory '"/nix/store/46p1z0w9ad605kky62dr53z4h24k2a5r-nix-2.25.2/bin/nix"' does not appear to be part of a Nix profile
```

This seems to happen for two reasons:

1. The original PATH search resulted in a directory, but `find` returns
   the path to the executable. Fixed by getting the path's parent.
2. The profile symlink cannot be found because
   `ExecutablePath::load().find` canonicalizes the executable path. I
   updated find to normalize the path instead, which seems more in line
   with how other programs resolve paths. I'm not sure if this affects
   other callers though.

I manually tested this on macOS and Linux, and it seemed to fix
upgrading from 2.25.2 to 2.25.3.

src/libutil/executable-path.cc

@@ -73,7 +73,7 @@ ExecutablePath::findName(const OsString & exe, std::function<bool(const fs::path
     for (auto & dir : directories) {
         auto candidate = dir / exe;
         if (isExecutable(candidate))
-            return std::filesystem::canonical(candidate);
+            return candidate.lexically_normal();
     }
 
     return std::nullopt;

src/nix/upgrade-nix.cc

@@ -107,7 +107,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
         auto whereOpt = ExecutablePath::load().findName(OS_STR("nix-env"));
         if (!whereOpt)
             throw Error("couldn't figure out how Nix is installed, so I can't upgrade it");
-        auto & where = *whereOpt;
+        const auto & where = whereOpt->parent_path();
 
         printInfo("found Nix in '%s'", where);