cipd_bin_setup: load cipd script instead of running it

Run it in a subshell to avoid forking+execing another shell.

Bug: 1469696
Change-Id: I173f18aa3bf2f181db261750f4e51422717886ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4753234
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>

cipd_bin_setup.sh

@@ -15,8 +15,10 @@ function cipd_bin_setup {
         ;;
     esac
 
-    "$MYPATH/cipd" ensure \
+    (
+    source "$MYPATH/cipd" ensure \
         -log-level warning \
         -ensure-file "$ENSURE" \
         -root "$ROOT"
+    )
 }