Encode branch name when creating a Binder link

[jtpio]

Currently the PR_HEAD_REF is taken as is and passed to the Binder link:

maintainer-tools/.github/actions/binder-link/action.yml

Lines 19 to 21 in 45e603c

	var PR_HEAD_REF = process.env.PR_HEAD_REF;
	var URL_PATH = process.env.URL_PATH;
	var BODY = `[](https://mybinder.org/v2/gh/${PR_HEAD_USERREPO}/${PR_HEAD_REF}?urlpath=${URL_PATH}) :point_left: Launch a Binder on branch _${PR_HEAD_USERREPO}/${PR_HEAD_REF}_`;

However this can create issues when a branch has a special character, for example #, as noticed in jupyter/notebook#7178. The Binder link becomes https://mybinder.org/v2/gh/haok1402/notebook/issue#7147?urlpath=tree, but Binder fails to build it:

However Binder can build the repo fine if the special character is URL encoded: https://mybinder.org/v2/gh/haok1402/notebook/issue%237147

So this binder-link action should probably try to encode the PR_HEAD_REF before creating the link.