Enable FreeBSD Core-Setup builds

[dagood]

Due to https://github.com/dotnet/core-setup/issues/4949, the FreeBSD leg was disabled. The infra issues should be fixed now.

CoreFX needs to start building FreeBSD again so Core-Setup can pull in the packages: dotnet/corefx#34869.

PRs related to disabling:

• Remove FreeBSD temporarily: unblock official build core-setup#5035 (removes build leg)
• Fix FreeBSD leg removal core-setup#5036 (fixup for build leg removal)
• Update dependencies from dotnet/corefx core-setup#5055 (removes FreeBSD RID)

Related (not a prereq): Enabling FreeBSD in Core-SDK is tracked by dotnet/installer#248.

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When we have time to implement this on the core-setup side, the plan is to ping @safern to turn it on in corefx. (This may involve downgrading the version of core-setup that corefx uses to one that included the FreeBSD leg.) Then we turn on the core-setup FreeBSD leg, then corefx uptakes the new core-setup build.

[ghost]

Does 'Future' milestone mean that FreeBSD support is out of scope for .NET Core 3.0 and .NET Core 4.0 is more likely?

[dagood]

Yes, currently out of scope for 3.0.

One thing to clarify though: this issue is only about adding FreeBSD builds to Core-Setup, which wouldn't necessarily mean FreeBSD is supported. The issue's only waiting for someone to have time to do the dance to get FreeBSD builds enabled again. (We want to keep the FreeBSD work from rotting even if we don't support it yet.)

/cc @leecow

[NikolaMilosavljevic]

Stale issue - closing.