om health: Improve tracing message for flake URL

[Adda0]

This PR adds a missing parenthesis to a tracing message and, when at it, pretty prints the flake URL instead of leaking the Rust debug print of Some(FlakeUrl(...)) or None to the user. This output is consistent with the Flake field in the printed table below.
Previously (note the missing ending parenthesis):

$ om health
...
🩺 Checking the health of your Nix setup (flake=None
...

and

$ om health "github:juspay/nixos-unified-template"
...
🩺 Checking the health of your Nix setup (flake=Some(FlakeUrl(github:juspay/nixos-unified-template))
...

Now:

$ om health
...
🩺 Checking the health of your Nix setup (flake: 'N/A')
──────────────────────────
 Property    Value
──────────────────────────
 Flake       N/A
...

and

$ om health "github:juspay/nixos-unified-template"
...
🩺 Checking the health of your Nix setup (flake: 'github:juspay/nixos-unified-template')
──────────────────────────────────────────────────
 Property    Value
──────────────────────────────────────────────────
 Flake       github:juspay/nixos-unified-template
...

cargo test fails on some unrelated issue which fails on master as well.

[srid]

Thanks!