Checking if a variable already exists

Signed-off-by: Junior Santos <claudio.dossantosjunior@yahoo.com.br>

hack/release.sh

@@ -42,8 +42,14 @@ OFFICIAL_RELEASE=${OFFICIAL_RELEASE:-false}
 # LOCAL_RELEASE indicates that containerd has been built and released
 # locally.
 LOCAL_RELEASE=${LOCAL_RELEASE:-false}
+if [ -z "${GOOS:-}" ]
+then
     GOOS=$(go env GOOS)
+fi
+if [ -z "${GOARCH:-}" ]
+then
     GOARCH=$(go env GOARCH)
+fi
 
 
 destdir=${BUILD_DIR}/release-stage