When `cargo deny` commands are run from the top-level directory of
a workspace, and no `--manifest-path` option is pased, they
implicitly use the top-level `Cargo.toml` file as their manifest.

In workspaces where this is a virtual manifest, i.e. those where
the top-level `Cargo.toml` defines only the workspace and not also
a specific crate, running `cargo deny` without specifying any roots
implicitly acts as though `--workspace` has been passed. (See
https://embarkstudios.github.io/cargo-deny/cli/common.html for
details.) In that situation, `--workspace` can be omitted and all
crates in the workspace are still treated as roots for the scan.

But the gitoxide repository's top-level workspace's `Cargo.toml`
file is not a virtual manifest. It defines a workspace, but it also
defines the `gitoxide` crate. As a result, `cargo deny` commands,
as run on CI in the `cargo-deny` and `cargo-deny-advisories` jobs,
and locally via `just audit`, were not guaranteed to check the
entire workspace. They used only the `gitoxide` crate as a root for
scanning, rather than using all crates defined in the workspace as
roots as when `--workspace` is used or implied.

It looks like this was not detected for three reasons:

1. Most of the workspace was usually covered, especially on CI
   where, since no `arguments` were passed, `cargo-deny-action`
   used the defualt of `--all-features`. *Most* transitive
   dependencies inside and outside of the gitoxide project seem to
   be used by the `gitoxide` crate by in some feature
   configuration.

2. The distinction between virtual and non-virtual top-level
   manifests of workspaces is subtle and not highlighted in the
   `cargo deny` documentation.

3. In `EmbarkStudios/cargo-deny-action`, the default value of
   `arguments` contains `--all-features` but not `--workspace` (nor
   any other options). Intuitively it feels like the default value
   would scan all crates in the repository that the action is being
   run on. That does happen in the perhaps more common case that
   the top-level `Cargo.toml` defines no crate, but not here.

This was discovered in connection with GitoxideLabs#1924. It is why the
`cargo-deny-advisories` job didn't catch RUSTSEC-2025-0022
(GHSA-4fcv-w3qc-ppgg) even as Dependabot did.

To fix it, this adds `--workspace` explicitly in both `cargo deny`
jobs run on CI, as well as in the `justfile`.

This also adds `--all-features`:

- On CI, adding `--all-features` is itself no change from before,
  since it is the implicit value of `arguments`. It has to be
  passed explicitly now that `arguments` has an explicit value.

- In the `justfile`, adding `--all-features` does (at least in
  principle) make a difference.