Merge branch 'sg/ci-brew-gcc-workaround' into maint

Dev support update.

* sg/ci-brew-gcc-workaround:
  ci/lib.sh: update a comment about installed P4 and Git-LFS versions
  ci: disable Homebrew's auto cleanup
  ci: don't update Homebrew

ci/install-dependencies.sh

@@ -34,7 +34,7 @@ linux-clang|linux-gcc)
 	popd
 	;;
 osx-clang|osx-gcc)
-	brew update >/dev/null
+	export HOMEBREW_NO_AUTO_UPDATE=1 HOMEBREW_NO_INSTALL_CLEANUP=1
 	# Uncomment this if you want to run perf tests:
 	# brew install gnu-time
 	test -z "$BREW_INSTALL_PACKAGES" ||

ci/lib.sh

@@ -163,8 +163,10 @@ linux-clang|linux-gcc)
 	export GIT_TEST_HTTPD=YesPlease
 
 	# The Linux build installs the defined dependency versions below.
-	# The OS X build installs the latest available versions. Keep that
-	# in mind when you encounter a broken OS X build!
+	# The OS X build installs much more recent versions, whichever
+	# were recorded in the Homebrew database upon creating the OS X
+	# image.
+	# Keep that in mind when you encounter a broken OS X build!
 	export LINUX_P4_VERSION="16.2"
 	export LINUX_GIT_LFS_VERSION="1.5.2"