entrypoint: handle linux in pkgs/platforms

Without this, we can errors such as the following

    ERROR failed to get CPU variant os=linux error="getCPUVariant for OS linux: not implemented"
    could not find command for platform "linux/arm64"

The main reason for this is because we are missing the variant when
getting the platform, but the oci images for arm64 have it, and thus
`TEKTON_PLATFORM_COMMANDS` as well. It creates a mismatch, and the
entrypoint cannot find the command it looks for.

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

Author: vdemeester

pkg/platforms/platforms.go

@@ -36,6 +36,7 @@ const (
 	Windows = "windows"
 	Darwin  = "darwin"
 	FreeBSD = "freebsd"
+	Linux   = "linux"
 )
 
 // Platform describes the platform which the image in the manifest runs on.
@@ -121,7 +122,7 @@ func getCPUVariant() (string, error) {
 		default:
 			variant = Unknown
 		}
-	case FreeBSD:
+	case Linux, FreeBSD:
 		// FreeBSD supports ARMv6 and ARMv7 as well as ARMv4 and ARMv5 (though deprecated)
 		// detecting those variants is currently unimplemented
 		switch runtime.GOARCH {