Added a VENV option when building Tribler

Jenkins by default uses sh to run its scripts. However, sh does not have the source command, which is needed to enter a virtual environment. Therefore, I added a VENV option to the build script (on Linux).

build/debian/makedist_debian.sh

@@ -8,6 +8,11 @@ then
   echo "Please run this script from project root as:\n./build/debian/makedist_debian.sh"
 fi
 
+if [ ! -z "$VENV" ]; then
+  echo "Setting venv to $VENV"
+  source $VENV/bin/activate
+fi
+
 rm -rf build/tribler
 rm -rf dist/tribler
 rm -rf build/debian/tribler/usr/share/tribler
@@ -34,4 +39,4 @@ if [ -x "$(command -v docker)" ]; then
     cd build/debian && docker run -v "$PWD":/debian -w /debian triblertester/snap_builder:core18 /bin/bash ./build-snap.sh
 else
     cd build/debian && /bin/bash ./build-snap.sh
-fi
+fi