Add release zsync update files to continuous

As there will be releases 1.7.1, 1.7.2 etc., storing zsync in release
asset doesn't make sense (further version won't update previous update
information). Thus store this in continuous release assets - update
channel "release" was introduced aside to existing "continuous".

.github/scripts/environment.sh

@@ -5,6 +5,7 @@ NDI=--disable-ndi
 if expr $GITHUB_REF : 'refs/tags/'; then
   TAG=${GITHUB_REF#refs/tags/}
   VERSION=${TAG#v}
+  CHANNEL=release
 elif [ $GITHUB_REF = 'refs/heads/ndi-build' ]; then
   NDI=--enable-ndi
   VERSION=ndi
@@ -14,8 +15,13 @@ else
   TAG=$VERSION
 fi
 
-export NDI TAG VERSION
+if [ -z ${CHANNEL-""} ]; then
+        CHANNEL=$VERSION
+fi
 
+export CHANNEL NDI TAG VERSION
+
+echo "CHANNEL=$CHANNEL" >> $GITHUB_ENV
 echo "NDI=$NDI" >> $GITHUB_ENV
 echo "TAG=$TAG" >> $GITHUB_ENV
 echo "VERSION=$VERSION" >> $GITHUB_ENV