Invoke script directly on CI instead of via just

This speeds things up, as no cargo or just commands have to be run,
and no Rust toolchain has to be installed.

If part of all of the script is replaced by a tool implemented in
Rust, then of course this should (and probably must) be revisited.

.github/workflows/ci.yml

@@ -204,17 +204,13 @@ jobs:
         shell: bash
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@v1
-        with:
-          toolchain: stable
-      - uses: extractions/setup-just@v2
       - name: Check that working tree is initially clean
         run: |
           git status
           set -x
           git status | grep -qF 'nothing to commit, working tree clean'
       - name: Regenerate gix-packetline-blocking/src
-        run: just copy-packetline
+        run: etc/copy-packetline.sh
       - name: Check that gix-packetline-blocking/src was already up to date
         run: |
           git status