We have name fragmentation for this feature, including "shared GC",
"modular GC", and "external GC". This commit standardizes the feature
name to "modular GC" and the implementation to "GC library".

Co-Authored-By: Peter Zhu <peter@peterzhu.ca>

Instead of passing the full GC SO file name to RUBY_GC_LIBRARY, we now
only need to pass the GC name.

For example, before we needed to pass `RUBY_GC_LIBRARY=librubygc.default.so`
but now we only need to pass `RUBY_GC_LIBRARY=default`.

Since `github.event.pull_request.labels.*.name` is an object filter,
the item to be searched in needs to be the same whole string, not a
partial string.