MFH: r434316

ports_env: Fix OPSYS not being exported.

This was causing excessive 'uname -s' executions during Poudriere dependency
calculations. The reasoning is due to HAVE_COMPAT_IA32_KERN having a newline
quoted at the end of it as '\ ' by bmake:

1. make -f Mk/bsd.port.mk -V 'HAVE_COMPAT_IA32_KERN=${HAVE_COMPAT_IA32_KERN:Q}' | cat -v HAVE_COMPAT_IA32_KERN=YES\

This resulted in the following being executed:

export HAVE_COMPAT_IA32_KERN="YES OPSYS=FreeBSD"

With hat: portmgr
Approved by: portmgr (implicit)