Fix platform detection for FreeBSD

[Thefrank]

Fix platform detection for FreeBSD

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Accept FreeBSD as a valid OS without need for --os-name

Description

The current version of eng/build.sh only checks for "Darwin" in uname's return and assumes "linux" otherwise. Under normal circumstances this is fine as building under FreeBSD we use --os-name FreeBSD to override this behavior. Unfortunately this is not possible when source building.

Fixes #47838

[ghost]

Thanks for your PR, @Thefrank. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[ghost]

Hey @dotnet/aspnet-build, looks like this PR is something you want to take a look at.

[wtgodbe]

Thanks for the change!