ci: don't install cmake on macOS as it's already installed

It seems `cmake` is already installed on macOS,
which makes `brew install cmake` fail, so don't try to re-install it.

Author: kkysen

.github/workflows/ci.yml

@@ -90,8 +90,9 @@ jobs:
       - name: Install packages (macOS)
         if: runner.os == 'macOS'
         run: |
-          # `bash` needed b/c macOS ships with bash 3, which doesn't support arrays properly
-          brew install -q cmake ninja gpg llvm@${{ matrix.clang-version }} bash
+          # `cmake` needed, but should be already installed.
+          # `bash` needed b/c macOS ships with bash 3, which doesn't support arrays properly.
+          brew install -q ninja gpg llvm@${{ matrix.clang-version }} bash
 
       - name: cargo build --release
         run: |