build/deb: fix PPA env var setting

build/deb/ethereum-swarm/deb.rules

@@ -5,7 +5,7 @@
 #export DH_VERBOSE=1
 
 # Launchpad rejects Go's access to $HOME/.cache, use custom folder
-export GOCACHE /tmp/go-build
+export GOCACHE=/tmp/go-build
 
 override_dh_auto_build:
 	build/env.sh /usr/lib/go-1.11/bin/go run build/ci.go install -git-commit={{.Env.Commit}} -git-branch={{.Env.Branch}} -git-tag={{.Env.Tag}} -buildnum={{.Env.Buildnum}} -pull-request={{.Env.IsPullRequest}}

build/deb/ethereum/deb.rules

@@ -5,7 +5,7 @@
 #export DH_VERBOSE=1
 
 # Launchpad rejects Go's access to $HOME/.cache, use custom folder
-export GOCACHE /tmp/go-build
+export GOCACHE=/tmp/go-build
 
 override_dh_auto_build:
 	build/env.sh /usr/lib/go-1.11/bin/go run build/ci.go install -git-commit={{.Env.Commit}} -git-branch={{.Env.Branch}} -git-tag={{.Env.Tag}} -buildnum={{.Env.Buildnum}} -pull-request={{.Env.IsPullRequest}}