fix CI job for building on MSRV

It turns out that dtolnay/rust-toolchain sets the installed toolchain
as the global default, but rustup prioritizes our rust-toolchain.toml.
Instead, set an environment variable. RUSTUP_TOOLCHAIN has a higher
priority than rust-toolchain.toml [^1].

[^1]: https://rust-lang.github.io/rustup/overrides.html#overrides

Author: Freax13

.github/workflows/build.yml

@@ -26,6 +26,9 @@ jobs:
           - nightly
           - 1.59
     runs-on: ubuntu-latest
+    env:
+      # rustup prioritizes environment variables over rust-toolchain.toml files.
+      RUSTUP_TOOLCHAIN: ${{ matrix.rust }}
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master