Fix PECL problem

[veblush]

Currently distribtest.php7_macos_x64_None is failing. This is believed to be caused by Homebrew bug which fails to install pecl correctly on MacOS.

running: make INSTALL_ROOT="/private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev" install
Installing shared extensions:     /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr/local/Cellar/php@7.2/7.2.28/pecl/20170718/
running: find "/private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev" | xargs ls -dils
10548148     0 drwxr-xr-x  3 root  wheel       96 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev
10555091     0 drwxr-xr-x  3 root  wheel       96 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr
10555092     0 drwxr-xr-x  3 root  wheel       96 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr/local
10555093     0 drwxr-xr-x  3 root  wheel       96 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr/local/Cellar
10555094     0 drwxr-xr-x  3 root  wheel       96 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr/local/Cellar/php@7.2
10555095     0 drwxr-xr-x  3 root  wheel       96 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr/local/Cellar/php@7.2/7.2.28
10555096     0 drwxr-xr-x  3 root  wheel       96 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr/local/Cellar/php@7.2/7.2.28/pecl
10555097     0 drwxr-xr-x  3 root  wheel       96 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr/local/Cellar/php@7.2/7.2.28/pecl/20170718
10555098 16584 -rwxr-xr-x  1 root  wheel  8488844 Dec  4 22:53 /private/tmp/pear/temp/pear-build-rootoQjx91/install-grpc-1.35.0dev/usr/local/Cellar/php@7.2/7.2.28/pecl/20170718/grpc.so

Build process completed successfully
Installing '/usr/local/Cellar/php@7.2/7.2.28/pecl/20170718/grpc.so'

Warning: mkdir(): File exists in System.php on line 294
PHP Warning:  mkdir(): File exists in /usr/local/Cellar/php@7.2/7.2.28/share/php@7.2/pear/System.php on line 294

Warning: mkdir(): File exists in /usr/local/Cellar/php@7.2/7.2.28/share/php@7.2/pear/System.php on line 294
ERROR: failed to mkdir /usr/local/Cellar/php@7.2/7.2.28/pecl/20170718

Related to Homebrew/homebrew-core#41081

[veblush]

grpc_build_artifacts_multiplatform: passed (build_package.csharp_package_unity_linux failed but it's not relevant)

[stanley-cheung]

LGTM. Thanks!

[jtattermusch]

Merging to fix the breakage.