ci: Remove xtask from MSRV build

We don't care about the MSRV for xtask, since it's just a dev tool. Also, some
of xtask's dependencies (like clap) update MSRV somewhat frequently, so in order
to update to the latest version of those deps it's better to not restrict
xtask's MSRV.

Author: nicholasbishop

.github/workflows/rust.yml

@@ -114,7 +114,10 @@ jobs:
         run: cp .github/workflows/msrv_toolchain.toml rust-toolchain.toml
       - uses: Swatinem/rust-cache@v2
       - name: Build
-        run: cargo xtask build
+        # Build uefi-test-runner since its dependency tree includes all the
+        # library packages. Note that xtask isn't used or built here; since it's
+        # just a dev tool we don't care about the MSRV for that package.
+        run: cargo build --target x86_64-unknown-uefi -p uefi-test-runner
   # This job requires the nightly channel, but keep it as a separate job from
   # `nightly_channel` because it takes a while to run.
   build_feature_permutations: