ci: pass -Zdoctest-xcompile to cargo test to run doctests

By default, cargo test does not run doctests when the target triple does
not match the host triple. There are some valid reasons for that to be
the case, but we didn't notice until now because no warnings were
outputted: the doctests were just silently skipped.

Let's use unstable Rust features to run those tests as originally
intended.

Related tracking issue: rust-lang/rust#64245

.github/workflows/ci.yml

@@ -104,4 +104,5 @@ jobs:
           # Handle AArch64 dynamic libraries installed by libc6-arm64-cross
           # being in a path that the dynamic linker does not check by default
           LD_LIBRARY_PATH: /usr/aarch64-linux-gnu/lib
-        run: cargo test --release --target ${{ matrix.target }} ${{ matrix.packages-to-test }} -- --nocapture
+        run: cargo test --release --target ${{ matrix.target }} -Zdoctest-xcompile
+          ${{ matrix.packages-to-test }} -- --nocapture