CI: make sure the target toolchain is available

[mjpieters]

For cross-compilation builds, the target toolchain needs to be installed (you
can't count on it being present in a GH build environment).

Making this change (adding the target: ${{ matrix.target }} line to the
actions-rs/toolchain@v1 action), make my PR #320 build properly for the iOS
cross-compilation target ^*). Without it, you may get errors like:

error[E0463]: can't find crate for `core`
Error:   |
  = note: the `aarch64-apple-ios` target may not be installed
  = help: consider downloading the target with `rustup target add aarch64-apple-ios`

With the target line rustup makes sure to add any missing toolchains; e.g. the iOS Setup Rust toolchain step now includes:

/Users/runner/.cargo/bin/rustup target add --toolchain stable aarch64-apple-ios
info: downloading component 'rust-std' for 'aarch64-apple-ios'
info: installing component 'rust-std' for 'aarch64-apple-ios'

If you were lucky and got a build machine that already had the toolchain installed, the build would pass, I think.

Note: I omitted a changelog entry as this is purely a CI fix.

Checklist

• I picked the correct source and target branch.
• I included a new entry to the CHANGELOG.md.
• I checked cargo clippy and cargo fmt. The CI will fail otherwise anyway.
• (If applicable) I added tests for this feature or adjusted existing tests.
• (If applicable) I checked if anything in the wiki needs to be changed.

---

^*) The aarch64-apple-ios build was cancelled because of an unrelated issue with a pre-existing clippy warning, but the build got well past the point where otherwise it'd have failed.

[mjpieters]

Can I suggest that the .github folder is also excluded from codecov? ;-)

codecov/patch — 0.00% of diff hit (target 53.38%)

[Nukesor]

Nice. This looks good to me :) Thanks for taking the time and fixing this!