Fix docs build (rustsec#871)

Workaround for rust-lang/rust#109266

This pins the docs build to MSRV, which should prevent regressions where
new releases break the docs build.

Author: tarcieri

.github/workflows/cargo-audit.yml

@@ -101,7 +101,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
+          toolchain: 1.60.0 # MSRV
           override: true
           profile: minimal
       - run: cargo doc --all-features

.github/workflows/rustsec.yml

@@ -46,7 +46,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
+          toolchain: 1.60.0 # MSRV
           override: true
           profile: minimal
       - run: cargo doc --all-features