Match Earthdata secrets with earthaccess environment variables

[scottyhq]

the widely used earthaccess library uses specific environment variables, so we should change these:

jupyterbook-template/.github/workflows/manual.yaml

Lines 17 to 18 in dbd9be6

	EARTHDATA_USER: ${{ secrets.EARTHDATA_USER}}
	EARTHDATA_PASS: ${{ secrets.EARTHDATA_PASS }}

xref: snowex-hackweek/website-2024#11 (comment)

[scottyhq]

(or just remove altogether if we want this template to be generic and not NASA specific...)