CI: Really apply buildcache on macos

Xcode uses
/Applications/Xcode_10.3.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
by default. buildcache uses a wrapper for clang/clang++ and the action
prepends the wrapper to the PATH. But in order to use it, clang needs to
be called by its name alone, without full path.

.github/workflows/ci.yml

@@ -140,7 +140,7 @@ jobs:
           cache_key: ${{ matrix.platform }}
       - name: Build scummvm
         run: |
-          xcodebuild build -project scummvm.xcodeproj ${{ matrix.buildFlags }}
+          xcodebuild CC=clang CPLUSPLUS=clang++ LD=clang LDPLUSPLUS=clang++ build -project scummvm.xcodeproj ${{ matrix.buildFlags }}
           ls
   ubuntu:
     name: Ubuntu