github: build and test all targets

It seems that at least examples are not included in the default set of
targets, and we clearly want to check that the examples compile, as
that's an important reason we have them. We don't have any tests for
the examples yet, but let's add the flag now so we don't forget it
later.

.github/workflows/build.yml

@@ -36,9 +36,9 @@ jobs:
       with:
         toolchain:  ${{ matrix.rust_version }}
     - name: Build
-      run: cargo build --workspace --verbose
+      run: cargo build --workspace --all-targets --verbose
     - name: Test
-      run: cargo test --workspace --verbose
+      run: cargo test --workspace --all-targets --verbose
       env:
         RUST_BACKTRACE: 1