Pin cargo-platform dependency

Cargo-platform no longer works on rustc older than 1.78, whereas skeptic
still does not work on rustc newer than 1.76.

    error: package `cargo-platform v0.1.9` cannot be built because it requires
    rustc 1.78 or newer, while the currently active rustc version is 1.76.0
    Either upgrade to rustc 1.78 or newer, or use
    cargo update [email protected] --precise ver
    where `ver` is the latest version of `cargo-platform` supporting rustc 1.76.0

Author: dtolnay

.github/workflows/ci.yml

@@ -24,6 +24,9 @@ jobs:
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{matrix.rust}}
+      - name: pin dependencies
+        run: cargo update cargo-platform --precise 0.1.8
+        working-directory: _skeptic
       - run: cargo test
         working-directory: _skeptic
       - uses: actions/upload-artifact@v4