[ci] Use pinned nightly for cargo-semver-checks (#753)

As reported in obi1kenobi/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: