ci: use StepSecurity's maintained actions (#7026)

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/fuzz.yml

@@ -31,10 +31,10 @@ jobs:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
-      - uses: dtolnay/rust-toolchain@686976e191b89faba57d3206551f0f330d8cb249 # stable
+      - uses: step-security/dtolnay-rust-toolchain@8dd607977d06adf4ed49b190381e74c17a9dd25f # v1.0.0
         with:
           toolchain: stable
-      - uses: Swatinem/rust-cache@d1527c8f67ef9af357290475d4af4b0fab8f12e0 # v2.9.3
+      - uses: step-security/rust-cache@8871c8ba063416da7c32246932ea6ae08700453e # v2.9.2
         with:
           key: fuzz
       - name: Fetch the pdf.js corpus (seeds)