tests/ci: only install qemu-user-static if QEMUSTATIC is set

In an upcoming patch, arm64 hosts will default to using bhyve to run CI. 
However, emulators/qemu-user-static does not exist for arm64, and is not needed 
when the target is also arm64.

This patch ensures that qemu-user-static will only be requested for install if QEMUSTATIC is set.

Signed-off-by: Siva Mahadevan <[email protected]>
Sponsored by: The FreeBSD Foundation

Author: svmhdvn

tests/ci/Makefile

@@ -151,7 +151,7 @@ portinstall-pkg: .PHONY
 .endif
 
 portinstall-qemu: portinstall-pkg .PHONY
-.if !exists(/usr/local/bin/qemu-${QEMU_ARCH}-static)
+.if !empty(QEMUSTATIC) && !exists(${QEMUSTATIC})
 	env ASSUME_ALWAYS_YES=yes pkg install emulators/qemu-user-static
 .endif
 .if !exists(${QEMUBIN})