setup & share current uname -s value

Avoid recomputing this value multiple times between vpython & cipd.

Bug: 1469696
Change-Id: If82d58180f94cb2833a5a177f7b1e431e08f2892
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4750040
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/vpython b/vpython
index 08ed05b..42d112f 100755
--- a/vpython
+++ b/vpython
@@ -37,12 +37,13 @@
 
 # Export for other depot_tools scripts to re-use.
 export DEPOT_TOOLS_DIR="${DEPOT_TOOLS_DIR:-$(dirname "$0")}"
+export DEPOT_TOOLS_UNAME_S="${DEPOT_TOOLS_UNAME_S:-$(uname -s | tr '[:upper:]' '[:lower:]')}"
 
 source "${DEPOT_TOOLS_DIR}/cipd_bin_setup.sh"
 cipd_bin_setup &> /dev/null
 
-case $(uname -s) in
-MINGW*|CYGWIN*)
+case "${DEPOT_TOOLS_UNAME_S}" in
+mingw*|cygwin*)
   cmd.exe //c $0.bat "$@"
   ;;
 *)