ci: Change MSRV job to read MSRV from cargo

Delete msrv_toolchain.toml, and instead read the MSRV from cargo and copy it to
rust-toolchain.toml. This ensures the job is checking the same MSRV set in
Cargo.toml, and reduces the number of places that the MSRV needs to be manually
updated when we change it.

Author: nicholasbishop

.github/workflows/msrv_toolchain.toml

@@ -120,7 +120,12 @@ jobs:
       - name: Checkout sources
         uses: actions/checkout@v4
       - name: Set toolchain
-        run: cp .github/workflows/msrv_toolchain.toml rust-toolchain.toml
+        run: |
+          # Extract the MSRV using cargo.
+          msrv=$(cargo metadata --no-deps --format-version=1 | jq --raw-output '.packages[] | select(.name == "uefi") | .rust_version')
+          echo "MSRV: ${msrv}"
+          # Set the MSRV in the toolchain config.
+          sed -i "s:stable:${msrv}:" rust-toolchain.toml
       - uses: Swatinem/rust-cache@v2
       - name: Build
         # Build uefi-test-runner since its dependency tree includes all the