Fix cargo dinghy attempting to run objc2-proc-macro tests

These are compiled for the host system, and hence fail when trying to execute them in the simulator

See sonos/dinghy#144

.github/workflows/ci.yml

@@ -165,8 +165,8 @@ jobs:
           ~/extern/lib
           ~/extern/include
           ~/extern/sdk
-        # Change this key if we start caching more things
-        key: ${{ matrix.name }}-extern-v1
+        # Change this key if we start caching something different
+        key: ${{ matrix.name }}-extern-v3
 
     - name: Setup environment
       # These add to PATH-like variables, so they can always be set
@@ -268,7 +268,7 @@ jobs:
 
     - name: Install Cargo Dinghy
       if: matrix.dinghy && steps.extern-cache.outputs.cache-hit != 'true'
-      run: cargo install cargo-dinghy --version=^0.4 --root=$HOME/extern --target=x86_64-apple-darwin
+      run: cargo install cargo-dinghy --version=^0.4.68 --root=$HOME/extern --target=x86_64-apple-darwin
 
     - name: Run Cargo Dinghy
       if: matrix.dinghy