CI: Change MSRV test not using metric

Using metric will cause github setting page show it as
`build_on_msrv(1.77)`. It will require admin to change github setting
page when we bump MSRV.

Change it to hard coded number, so github setting page will how as
`build_on_msrv` only.

Signed-off-by: Gris Ge <[email protected]>

Author: cathay4t

.github/workflows/msrv.yml

@@ -14,17 +14,14 @@ jobs:
   build_on_msrv:
     strategy:
       fail-fast: true
-      matrix:
-        include:
-          - msrv: "1.77"
 
     runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v5
 
-    - name: Install Rust ${{ matrix.msrv }}
-      run: rustup install ${{ matrix.msrv }}
+    - name: Install Rust 1.77
+      run: rustup install 1.77
 
     - name: Build test
-      run: cargo +${{ matrix.msrv }} build --ignore-rust-version
+      run: cargo +1.77 build --ignore-rust-version