Microsoft.NETCore.Platforms: support adding rids with '-' in the base part. (#84413)

Microsoft.NETCore.Platforms: support adding rids with '-' in the base part. (#84413)

* Microsoft.NETCore.Platforms: support adding rids with '-' in the base part.

Currently when trying to add a rid like 'linux-musl-x64'
the rid is not understood to be base = 'linux-musl', arch = 'x64'.

Instead the parser considers a potential optional qualifier.
This causes the rid to be parsed as base = 'linux', arch = 'musl',
and qualifier = 'x64'.

We know the rids being added won't have a qualifier. If we take
this into account while parsing, we can parse the rid correctly.

* Update src/libraries/Microsoft.NETCore.Platforms/src/RuntimeGroupCollection.cs

Co-authored-by: Eric StJohn <ericstj@microsoft.com>
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Co-authored-by: Eric StJohn <ericstj@microsoft.com>