add isunix for BinaryPlatforms AbstractPlatform (#47543)

This provides symmetry with the methods defined in `Sys`,
and is convenient for use in e.g. BinaryBuilder platform filters.
Note that there are other `is.*bsd` variants we omit since they
are unsupported by BinaryPlatforms. The capitilization
conventions are also different so we do not call the Sys methods
directly.

base/binaryplatforms.jl

@@ -584,6 +584,7 @@ Sys.islinux(p::AbstractPlatform) = os(p) == "linux"
 Sys.iswindows(p::AbstractPlatform) = os(p) == "windows"
 Sys.isfreebsd(p::AbstractPlatform) = os(p) == "freebsd"
 Sys.isbsd(p::AbstractPlatform) = os(p) ∈ ("freebsd", "macos")
+Sys.isunix(p::AbstractPlatform) = Sys.isbsd(p) || Sys.islinux(p)
 
 const arch_mapping = Dict(
  "x86_64" => "(x86_|amd)64",