tools/ci/website_build.sh isn't safe to run locally

[gsnedders]

Historically @jgraham has cared a lot about making it easy to replicate what CI does locally, and often suggested moving things into scripts so that they can be easily tested locally.

For one, it fails to actually fetch the current gh-pages branch, because environment variables aren't set:

wpt/tools/ci/website_build.sh

Line 13 in d2f2efe

remote_url=https://${DEPLOY_TOKEN}@github.com/${GITHUB_REPOSITORY}.git

And for another, it changes the global git config:

wpt/tools/ci/website_build.sh

Lines 29 to 30 in d2f2efe

	git config --global user.email "wpt-pr-bot@users.noreply.github.com"
	git config --global user.name "wpt-pr-bot"

Before finally failing when it tries to read a file that doesn't exist:

wpt/tools/ci/website_build.sh

Lines 16 to 27 in d2f2efe

	function json_property {
	cat ${1} | \
	python -c "import json, sys; print(json.load(sys.stdin).get(\"${2}\", \"\"))"
	}
	function is_pull_request {
	test -n "$(json_property ${GITHUB_EVENT_PATH} pull_request)"
	}
	function targets_master {
	test $(json_property ${GITHUB_EVENT_PATH} ref) == 'refs/heads/master'
	}

Maybe there will be some disagreement here, but it feels like a lot of these (especially the conditionals at the end!) should just be individual workflow steps.