Use it check-mode instead of check-mode.sh; test on CI

- Delete the old `etc/check-mode.sh` script.
- Update `justfile` recipe `check-mode` to use internal-tools.
- Modify and expand `check-mode` CI job to use internal-tools.
- Temporarily make `check-mode` job a matrix job to test platforms.

The latter of those changes should be undone once things looks like
they are working. I have manually tested `it check-mode` on Arch
Linux and Windows in the same ways `check-mode.sh` was tested (and
unlike with a shell-script based approach, it is not anticipated to
differ on macOS). So trying three platforms on CI is a secondary
supporting strategy to catch possible problems.

.github/workflows/ci.yml

@@ -360,12 +360,18 @@ jobs:
         run: cd gix-pack && cargo build --all-features --target "$TARGET"
 
   check-mode:
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ ubuntu-latest, macos-latest, windows-latest ]
+
+    runs-on: ${{ matrix.os }}
 
     steps:
       - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: extractions/setup-just@v2
       - name: Find scripts with mode/shebang mismatch
-        run: etc/check-mode.sh
+        run: just check-mode
 
   check-packetline:
     strategy:

justfile

@@ -196,6 +196,7 @@ target_dir := `cargo metadata --format-version 1 | jq -r .target_directory`
 ein := target_dir / "debug/ein"
 gix := target_dir / "debug/gix"
 jtt := target_dir / "debug/jtt"
+it := target_dir / "debug/it"
 
 # run journey tests (max)
 journey-tests:
@@ -257,7 +258,8 @@ find-yanked:
 
 # Find shell scripts whose +x/-x bits and magic bytes (e.g. `#!`) disagree
 check-mode:
-    ./etc/check-mode.sh
+    cargo build -p internal-tools
+    {{ it }} check-mode
 
 # Delete gix-packetline-blocking/src and regenerate from gix-packetline/src
 copy-packetline: