ci: use StepSecurity's maintained actions (#23)

Each third-party action that StepSecurity publishes a secure drop-in
replacement for now uses that replacement, pinned to a commit with its
version as a comment. Versions follow what we used: the newest
StepSecurity release in the same major, or the closest major they
publish. dtolnay/rust-toolchain refs that named a toolchain (stable,
nightly, 1.76) become the replacement's `toolchain` input.

.github/workflows/gdi.yml

@@ -27,8 +27,9 @@ jobs:
         with:
           egress-policy: audit
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: step-security/dtolnay-rust-toolchain@8dd607977d06adf4ed49b190381e74c17a9dd25f # v1.0.0
         with:
+          toolchain: stable
           components: rustfmt, clippy
           targets: wasm32-unknown-unknown
       - run: cargo fmt --manifest-path crates/gdi/Cargo.toml --check