_build.sh: use CMAKE_IGNORE_PREFIX_PATH for macOS [ci skip]

CMake has a feature to search `/usr/local` by default for dependencies.
Setting the SDK root is not suppressing this behaviour. Use this setting
instead for that purpose.

Ref: https://cmake.org/cmake/help/latest/variable/CMAKE_IGNORE_PREFIX_PATH.html

_build.sh

@@ -1185,8 +1185,10 @@ build_single_target() {
     # Minimum SDK version supported by Xcode releases:
     #   https://developer.apple.com/support/xcode/
 
-    # Explicitly set the SDK root. This forces clang to drop /usr/local
-    # from the list of default header search paths.
+    # Do not pick up components from these prefixes:
+    _CMAKE_GLOBAL+=" -DCMAKE_IGNORE_PREFIX_PATH=/usr/local;${brew_root}"
+
+    # Explicitly set the SDK root.
     # We set it for all build tools for macOS to gain control over this.
     _SYSROOT="$(xcrun -sdk macosx --show-sdk-path)"  # E.g. /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk