Allow disabling build users by unsetting `build-users-group`

Unsetting `build-users-group` (without `auto-allocate-uids` enabled)
gives the following error:

```
src/libstore/lock.cc:25: static std::unique_ptr<nix::UserLock> nix::SimpleUserLock::acquire(): Assertion `settings.buildUsersGroup != ""' failed.
```

Fix the logic in `useBuildUsers` and document the default value
for `build-users-group`.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index 54a5d0fc7..274a15dd7 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -281,7 +281,10 @@ public:
           `NIX_REMOTE` is empty, the uid under which the Nix daemon runs if
           `NIX_REMOTE` is `daemon`). Obviously, this should not be used in
           multi-user settings with untrusted users.
-        )"};
+
+          Defaults to `nixbld` when running as root, *empty* otherwise.
+        )",
+        {}, false};
 
     Setting<bool> autoAllocateUids{this, false, "auto-allocate-uids",
         R"(