debian: add missing env variables to debian build

During the build of heartwood there are a few environment variables in
particular `GIT_HEAD` and `RADICLE_VERSION` that are added during build
to be part of the final binary.

This wasn't the case so far with the debian packages, this patch exports
the needed vars for `override_dh_auto_build` so commands like `rad
--version` display the values.

debian/rules

@@ -1,6 +1,8 @@
 #!/usr/bin/make -f
 
 DEB_BUILD_OPTIONS=nostrip
+export RADICLE_VERSION=$(shell build/version)
+export GIT_HEAD=$(shell git rev-parse HEAD)
 
 %:
 	dh $@