optimize README.md

Author: loongs-zhang

.github/workflows/coverage.yml

@@ -11,14 +11,12 @@ on:
       - '**.png'
 
 env:
-  RUST_TOOLCHAIN: stable
-  TOOLCHAIN_PROFILE: minimal
+  CARGO_TERM_COLOR: always
 
 jobs:
   coverage:
     name: Run cargo coverage
     runs-on: ubuntu-latest
-    environment: coverage
     env:
       CARGO_TERM_COLOR: always
     steps:
@@ -27,8 +25,8 @@ jobs:
       - name: Install toolchain
         uses: actions-rs/toolchain@v1
         with:
-          profile: ${{ env.TOOLCHAIN_PROFILE }}
-          toolchain: ${{ env.RUST_TOOLCHAIN }}
+          toolchain: 1.81.0
+          profile: minimal
           override: true
           components: llvm-tools-preview
       - name: Install cargo-llvm-cov