[ci] Use pinned nightly for cargo-semver-checks

As reported in cargo-semver-checks-action#61, a recent update to the
nightly toolchain broke cargo-semver-checks. Previously, we were using
the latest nightly toolchain rather than the pinned nightly toolchain
in order to run cargo-semver-checks, which caused our CI to break when
this new nightly was released.

In this commit, we use our pinned nightly version instead, hopefully
avoiding issues like this in the future.

.github/workflows/ci.yml

@@ -251,10 +251,7 @@ jobs:
         # an API that cargo-semver-checks can understand.
         package: zerocopy
         feature-group: all-features
-        # TODO: Set this to the specific nightly we have pinned in CI. Not a big
-        # deal since this isn't affected by the trybuild stderr files, which is
-        # the reason we need to pin to a specific nightly.
-        rust-toolchain: nightly
+        rust-toolchain: ${{ env.ZC_TOOLCHAIN }}
       if: matrix.crate == 'zerocopy' && matrix.features == '--all-features' && matrix.toolchain == 'nightly'
 
   kani: