Try force install Python 3.7

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -88,12 +88,10 @@ then
   time pip install --user --upgrade Mako tox setuptools==44.1.1 twisted pyyaml pyjwt cryptography requests
 
   # Install Python 3.7 if it doesn't exist
-  if [ ! -f "/usr/local/bin/python3.7" ]; then
-    time curl -O https://www.python.org/ftp/python/3.7.0/python-3.7.0-macosx10.9.pkg
-    echo "ee4ad46ab8cd226ffc8df56d48acfdf7daa2714a9c51e6dc6262fc0b25519578  python-3.7.0-macosx10.9.pkg" > /tmp/python_installer_checksum.sha256
-    shasum -c /tmp/python_installer_checksum.sha256
-    time sudo installer -pkg ./python-3.7.0-macosx10.9.pkg -target /
-  fi
+  time curl -O https://www.python.org/ftp/python/3.7.0/python-3.7.0-macosx10.9.pkg
+  echo "ee4ad46ab8cd226ffc8df56d48acfdf7daa2714a9c51e6dc6262fc0b25519578  python-3.7.0-macosx10.9.pkg" > /tmp/python_installer_checksum.sha256
+  shasum -c /tmp/python_installer_checksum.sha256
+  time sudo installer -pkg ./python-3.7.0-macosx10.9.pkg -target /
 
   # Install Python 3.8 if it doesn't exist
   if [ ! -f "/usr/local/bin/python3.8" ]; then