[build] Pass through any existing NINJA_STATUS in the env

Test: Ran `fx build` with and without a NINJA_STATUS set

Change-Id: I4ad3a6f2d1a8d25aecc5a80649db13a7341770f8

devshell/build

diff --git a/devshell/build b/devshell/build
index 279d83c..5489f46 100755
--- a/devshell/build
+++ b/devshell/build
@@ -57,6 +57,7 @@
   # when TMPDIR="" - it is deliberately unquoted and using the ${+} expansion
   # expression).
   fx-try-locked env -i TERM="${TERM}" PATH="${PATH}" \
+    ${NINJA_STATUS+"NINJA_STATUS=${NINJA_STATUS}"} \
     ${TMPDIR+"TMPDIR=$TMPDIR"} \
     "${FUCHSIA_DIR}/buildtools/ninja" -C "${FUCHSIA_BUILD_DIR}" "${args[@]}"
 }