Work around snap's old cabal + new snapcraft proxy.

snap/snapcraft.yaml

@@ -37,9 +37,16 @@ parts:
     source: ./
     build-packages:
       - cabal-install
+      - squid3
     build: |
+      # See comments in .snapsquid.conf
+      [ "$http_proxy" ] && {
+        squid3 -f .snapsquid.conf
+        export http_proxy="http://localhost:8888"
+        sleep 3
+      }
       cabal sandbox init
-      cabal update
+      cabal update || cat /var/log/squid/*
       cabal install -j
     install: |
       install -d $SNAPCRAFT_PART_INSTALL/usr/bin