explicitly use rust 1.74.0 in github actions

.github/workflows/pr.yml

@@ -12,24 +12,28 @@ jobs:
     - uses: actions/checkout@v2
     - uses: actions-rs/toolchain@v1
       with:
-        toolchain: stable
+        toolchain: 1.74.0
         components: rustfmt, clippy
     - uses: Swatinem/[email protected]
     - uses: actions-rs/cargo@v1
       with:
         command: fmt
+        toolchain: 1.74.0
         args: -- --check
     - uses: actions-rs/cargo@v1
       with:
         command: clippy
+        toolchain: 1.74.0
         args: -- --deny warnings
     - uses: actions-rs/cargo@v1
       with:
         command: install
+        toolchain: 1.74.0
         args: --locked cargo-deny
     - uses: actions-rs/cargo@v1
       with:
         command: deny
+        toolchain: 1.74.0
         args: check
   test:
     runs-on: ubuntu-20.04