CI: fix rebuilding peepmatic peephole optimizers (#2311)

The test that triggers the rebuild of the peephole optimizers is in the
`cranelift-codegen` crate, not the umbrella cranelift crate. This was previously
successfully running zero tests, and then successfully reporting no `git diff`
because no peephole optimizers were ever rebuilt.

This change fixes it so that we run the correct test that triggers the
rebuilding of the peephole optimizers.

.github/workflows/main.yml

@@ -168,9 +168,9 @@ jobs:
     - name: Rebuild Peepmatic-based peephole optimizers and test them
       run: |
         cargo test \
-          --features 'enable-peepmatic cranelift-codegen/rebuild-peephole-optimizers' \
+          --features 'enable-peepmatic rebuild-peephole-optimizers' \
           peepmatic
-      working-directory: ./cranelift
+      working-directory: ./cranelift/codegen
     - name: Check that built peephole optimizers are up to date
       run: git diff --exit-code
     - name: Test with Peepmatic-based peephole optimizers