ci: use an hardcoded version of cargo-machete in CI (matrix-org#4710)

Should resolve the CI issues around cargo-machete.

See also:
bnjbvr/cargo-machete#156 (comment)

Author: bnjbvr

.github/workflows/detect-unused-dependencies.yml

@@ -9,4 +9,4 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
       - name: Machete
-        uses: bnjbvr/cargo-machete@main
+        uses: bnjbvr/cargo-machete@v0.7.1