build: fix setting recipe env var when the var doesn't exists
build: fix setting recipe env var when the var doesn't exists

ford-prefect/cerbero,flexVDI/cerbero,fluendo/cerbero,EricssonResearch/cerbero,multipath-rtp/cerbero,atsushieno/cerbero,nirbheek/cerbero,cee1/cerbero-mac,EricssonResearch/cerbero,jackjansen/cerbero-2013,ikonst/cerbero,sdroege/cerbero,shoreflyer/cerbero,AlertMe/cerbero,superdump/cerbero,AlertMe/cerbero,ikonst/cerbero,jackjansen/cerbero-2013,nzjrs/cerbero,nirbheek/cerbero-old,nirbheek/cerbero,jackjansen/cerbero-2013,centricular/cerbero,ylatuya/cerbero,centricular/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,atsushieno/cerbero,fluendo/cerbero,ramaxlo/cerbero,BigBrother-International/gst-cerbero,davibe/cerbero,multipath-rtp/cerbero,jackjansen/cerbero-2013,brion/cerbero,superdump/cerbero,EricssonResearch/cerbero,nzjrs/cerbero,nzjrs/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,brion/cerbero,ikonst/cerbero,AlertMe/cerbero,davibe/cerbero,ford-prefect/cerbero,fluendo/cerbero,jackjansen/cerbero-2013,cee1/cerbero-mac,flexVDI/cerbero,jackjansen/cerbero,AlertMe/cerbero,cee1/cerbero-mac,superdump/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,davibe/cerbero,justinjoy/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,flexVDI/cerbero,OptoFidelity/cerbero,shoreflyer/cerbero,brion/cerbero,sdroege/cerbero,lubosz/cerbero,nzjrs/cerbero,ylatuya/cerbero,EricssonResearch/cerbero,ford-prefect/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,nicolewu/cerbero,fluendo/cerbero,nzjrs/cerbero,multipath-rtp/cerbero,atsushieno/cerbero,nirbheek/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,davibe/cerbero,jackjansen/cerbero,sdroege/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,ylatuya/cerbero,lubosz/cerbero,atsushieno/cerbero,GStreamer/cerbero,ford-prefect/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,atsushieno/cerbero,AlertMe/cerbero,GStreamer/cerbero,OptoFidelity/cerbero,centricular/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,sdroege/cerbero,ramaxlo/cerbero,GStreamer/cerbero,shoreflyer/cerbero,superdump/cerbero,GStreamer/cerbero,lubosz/cerbero,BigBrother-International/gst-cerbero,multipath-rtp/cerbero,flexVDI/cerbero,nirbheek/cerbero-old,ikonst/cerbero,cee1/cerbero-mac,sdroege/cerbero,flexVDI/cerbero,nirbheek/cerbero-old,freedesktop-unofficial-mirror/gstreamer__cerbero,justinjoy/cerbero,OptoFidelity/cerbero,ylatuya/cerbero,BigBrother-International/gst-cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,BigBrother-International/gst-cerbero,nicolewu/cerbero,ikonst/cerbero,justinjoy/cerbero,GStreamer/cerbero,EricssonResearch/cerbero,brion/cerbero,ramaxlo/cerbero,nicolewu/cerbero,lubosz/cerbero,shoreflyer/cerbero,centricular/cerbero,jackjansen/cerbero,justinjoy/cerbero,ramaxlo/cerbero,jackjansen/cerbero,brion/cerbero,ramaxlo/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,nirbheek/cerbero,multipath-rtp/cerbero,BigBrother-International/gst-cerbero,OptoFidelity/cerbero,shoreflyer/cerbero,centricular/cerbero,freedesktop-unofficial-mirror/gstreamer-sdk__cerbero,nirbheek/cerbero-old,fluendo/cerbero