[release/7.0-staging] Microsoft.NETCore.Platforms: support adding rids with '-' in the base part.

[github-actions]

Backport of #84413 to release/7.0-staging

/cc @carlossanlop @tmds

Per @tmds

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.

Customer impact

#77508 was merged, but its installer counterpart (dotnet/installer#14816) was NAKed due to safety concerns. This revealed a limit with Microsoft.NETCore.Platforms parsing of RID. Thus, build of source-build on linux-musl platforms is currently broken. A few approches on installer side can be implemented as workarounds, but ideally the fix should be on runtime side.

This also unblocks dotnet/installer#13074.

Testing

• unit tests in ci
• manual build of runtime on Alpine Linux when OutputRid=linux-musl-x64 and Source

Risk

Low, there were limited changes in that code between release/6.0 and main.

cc @tmds @ayakael

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Issue Details

---

Backport of #84413 to release/7.0-staging

/cc @carlossanlop @tmds

Customer Impact

Testing

Risk

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

• If the change touches code that ships in a NuGet package, you have added the necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	area-Infrastructure-libraries
Milestone:	-

[carlossanlop]

This change requires Microsoft.NETCore.Platforms.csproj OOB package authoring changes but they aren't needed in this PR if we merge this other one for the June release, which already contains such changes: #84984

[wfurt]

LGTM

[carlossanlop]

I'm treating this as tell-mode since the affected cs file is a build task, so not customer facing. I notified Tactics anyway just so they were informed.