[ci] Run cargo-semver-checks on all targets

Credit to @memark for landing obi1kenobi/cargo-semver-checks-action#82,
which unblocked this!

Closes #357

Author: joshlf

.github/workflows/ci.yml

@@ -340,11 +340,6 @@ jobs:
     # crates.io. We do this in the matrix rather than in its own job so that it
     # gets run on different targets. Some of our API is target-specific (e.g.,
     # SIMD type impls), and so we need to run on each target.
-    #
-    # TODO(https://github.com/obi1kenobi/cargo-semver-checks-action/issues/54):
-    # Currently we don't actually do anything with `matrix.target`, so we're
-    # just duplicating work by running this job multiple times, each time
-    # targetting the host platform.
     - name: Check semver compatibility
       uses: obi1kenobi/cargo-semver-checks-action@ca26a44cfb670b2078c8f757d06e696a7c3820cf # v2.5
       with:
@@ -363,6 +358,7 @@ jobs:
         feature-group: only-explicit-features
         features: __internal_use_only_features_that_work_on_stable
         rust-toolchain: ${{ env.ZC_TOOLCHAIN }}
+        rust-target: ${{ matrix.target }}
       if: matrix.crate == 'zerocopy' && matrix.features == '--features __internal_use_only_features_that_work_on_stable' && matrix.toolchain == 'stable'
 
     # TODO(#453): Doing this as a matrix step is a hack that allows us to depend