Work around new gix-index ambiguity for cargo check

The previous commit added `gix-testtools` (by relative path) as a
dev dependency of `gix-index`, to use `gix_testtools::size_ok`.
Because `gix-testtools` itself depends on `gix-index` -- at an
earlier version to not break releasing with csr (see discussion
in #1510 for general info) -- this causes `cargo`, when running in
the top level workspace directory, to consider `-p gix-index`
without an explicit version to be ambiguous.

This made the full CI `test` job fail when the `check` recipe
attempts to run `cargo check` on `gix-index`, with the message

    error: There are multiple `gix-index` packages in your project, and the specification `gix-index` is ambiguous.
    Please re-run this command with one of the following specifications:
      [email protected]
      [email protected]
    error: Recipe `check` failed on line 87 with exit code 101

where the line number is from the `justfile`.

To fix this, this changes the command to change to the `gix-index`
directory instead of passing `-p gix-index`. (This technique is
used elsewhere in the same recipe already.)

Author: EliahKagan

justfile

@@ -84,7 +84,7 @@ check:
     cargo check -p gix-pack --features object-cache-dynamic
     cargo check -p gix-packetline --features blocking-io
     cargo check -p gix-packetline --features async-io
-    cargo check -p gix-index --features serde
+    cd gix-index && cargo check --features serde
     cargo check -p gix-credentials --features serde
     cargo check -p gix-sec --features serde
     cargo check -p gix-revision --features serde