Enter JIRI_ROOT to run jiri update, some hooks need it

Some of the mojo hooks expect to run from the directory containing the Mojo
'src' directory since that's what gclient runhooks does. This needs to be
fixed, but for now this just cd's into the appropriate directory so that the
initial sync works.

Change-Id: Ic3da19150410df3330c262b6eb7857ee5191b0c0
diff --git a/bootstrap.sh b/bootstrap.sh
index 8554e7d..71e8645 100644
--- a/bootstrap.sh
+++ b/bootstrap.sh
@@ -85,6 +85,9 @@
   GOPATH="${JIRI_ROOT}/tmp" run go build -o "${JIRI_ROOT}/devtools/bin/jiri" github.com/vanadium/go.jiri
   rm -rf "${JIRI_ROOT}/tmp"
 
+  pushd .
+  cd "${JIRI_ROOT}"
+
   # Install all vanadium repositories and tools.
   local -r NATTEMPTS=3
   for attempt in $(seq 1 "${NATTEMPTS}"); do
@@ -100,6 +103,8 @@
 
   echo "Please add ${JIRI_ROOT}/devtools/bin and ${JIRI_ROOT}/depot_tools to your PATH."
 
+  popd
+
   trap - EXIT
 }