fix: use project-scoped artifact names, return test job result as workflow output

[matt-codecov]

two changes for https://github.com/codecov/umbrella/pull/12

first: umbrella now has a single workflow which runs worker, shared, and codecov-api workflows as subroutines. they, in turn, call these shared workflows which upload coverage and junit files with coveragefiles and junitfiles as their artifact names. worker and shared both try to use the same name and they fight. so, this change adds a prefix to the artifact name so there is no more conflict

second: each workflow now has an output variable that explicitly indicates whether tests passed. this is helpful for two reasons:

• when using run-tests-split, the workflow output is the simplest way to check whether any of the matrixed jobs failed
• we can make specifically the tests required and allow, like, the codecov upload jobs or whatever to fail if we want