use /tmp for the fuchsia build control paths

Some users have very long paths to $FUCHSIA_BUILD_DIR (100 characters or so),
and that's causing problems. We discussed other options, such as trying to
put these in $FUCHSIA_OUT_DIR, but this is the most reliable option for now.

Change-Id: I66fc26d6cb7d6386c2e34e209d90f4fc8ae4bae7
diff --git a/fuchsia/developer-keys/generate_ssh_config.sh b/fuchsia/developer-keys/generate_ssh_config.sh
index ecd8978..0f8fac5 100755
--- a/fuchsia/developer-keys/generate_ssh_config.sh
+++ b/fuchsia/developer-keys/generate_ssh_config.sh
@@ -11,5 +11,5 @@
   IdentityFile $1
   ControlPersist yes
   ControlMaster auto
-  ControlPath $(dirname $2)/%r@%h:%p
+  ControlPath /tmp/fuchsia-$USER-%r@%h:%p
 EOF