github: run only with MSRV, not stable

We run a separate build on CI with the MSRV so we notice if we
accidentally break the MSRV. However, as we talked about on Discord,
the opposite is very unlikely - that we accidentally break the build
with the stable release without breaking the MSRV build. Also, we
explicitly run Clippy with the stable release, and formatting with the
nightly release. So, let's just do the regular build and tests with
the MSRV.

.github/workflows/build.yml

@@ -18,23 +18,18 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-13, windows-latest]
-        rust_version: [stable]
         cargo_flags: [""]
         include:
         - os: ubuntu-latest
-          rust_version: "1.71"
-          cargo_flags: ""
-        - os: ubuntu-latest
-          rust_version: stable
           cargo_flags: "--all-features"
     runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@c85c95e3d7251135ab7dc9ce3241c5835cc595a9
-    - name: Install Rust (${{ matrix.rust_version }})
+    - name: Install Rust
       uses: dtolnay/rust-toolchain@0e66bd3e6b38ec0ad5312288c83e47c143e6b09e
       with:
-        toolchain:  ${{ matrix.rust_version }}
+        toolchain:  1.71
     - name: Build
       run: cargo build --workspace --all-targets --verbose ${{ matrix.cargo_flags }}
     - name: Test