CI: Remove most usages of `actions-rs` actions

These actions are unmaintained. The only one we continue to use is
`actions-rs/clippy-check` because there is no suitable alternative.

.github/workflows/audits.yml

@@ -11,20 +11,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: stable
-          override: true
-
-      - name: Install cargo-vet
-        uses: actions-rs/cargo@v1
-        with:
-          command: install
-          args: cargo-vet
-
-      - name: Run cargo vet --locked
-        uses: actions-rs/cargo@v1
-        with:
-          command: vet
-          args: --locked
+      - uses: dtolnay/rust-toolchain@stable
+        id: toolchain
+      - run: rustup override set ${{steps.toolchain.outputs.name}}
+      - run: cargo install cargo-vet
+      - run: cargo vet --locked

.github/workflows/book.yml

@@ -21,10 +21,7 @@ jobs:
           mdbook-version: 'latest'
 
       - name: Install mdbook-katex
-        uses: actions-rs/cargo@v1
-        with:
-          command: install
-          args: mdbook-katex
+        run: cargo install mdbook-katex
 
       - name: Build zcashd book
         run: mdbook build doc/book/

.github/workflows/lints.yml

@@ -75,19 +75,14 @@ jobs:
         if: always()
 
   rust-clippy:
-    name: Clippy (1.68.0)
+    name: Clippy (MSRV)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: 1.68.0
-          components: clippy
-          override: true
       - name: Run clippy
         uses: actions-rs/clippy-check@v1
         with:
-          name: Clippy (1.68.0)
+          name: Clippy (MSRV)
           token: ${{ secrets.GITHUB_TOKEN }}
           args: --all-features --all-targets -- -D warnings
 
@@ -96,12 +91,4 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: 1.68.0
-          override: true
-      - run: rustup component add rustfmt
-      - uses: actions-rs/cargo@v1
-        with:
-          command: fmt
-          args: -- --check
+      - run: cargo fmt -- --check

rust-toolchain.toml

@@ -1,2 +1,3 @@
 [toolchain]
 channel = "1.68.0"
+components = ["clippy", "rustfmt"]